Forums

Stable simple dc amplifiers

Started by Archival March 11, 2012
Archival wrote:

> On Monday, March 12, 2012 8:38:25 AM UTC-5, bloggs.fred...@gmail.com wrote: > >>On Sunday, March 11, 2012 5:32:01 PM UTC-4, Archival wrote: >> >>>I need a simple(= cheap) relatively stable(temp and noise) dc amplifier that can take a voltage from 0 to 5V and output from around 0V(not critical) to near Vcc(but arbitrary) somewhat linearly. >>> >>> >>>I'm thinking a simple bjt amplifier with temp compensation will work decently but the issue is linearity and range. >>> >>>e.g., the output voltage of an ideal CE amplifier is Vout = Vcc - Rc/Re*Vin >>> >>>Of course when Vin = 0 volts, Vout = Vcc. When Vin is 5V we get Vout = Vcc - 5Rc/Re but we would like 0V or some low fixed voltage instead. >>> >>>Adding temperature compensation makes things worse since it effects our upper range(which is more crucial than the lower range) since Vin cannot swing down to 0V(I'm assuming the simple diode compensation scheme generally used). >>> >>>In any case the requirements are >>> >>>1. Relatively cheap and easy to built(e.g., a few discrete components)/ >>>2. Amplifies a voltage from [0, Vin_max] to approximately [0, Vcc] with the upper range being more important. Vcc is somewhat arbitrary ==> May change after the design of the circuit ==> no component values can depend on Vcc to achieve specs. (Obviously one can assume that Vcc is within all the maximum voltage ratings of the components) >>>3. Temperature stable/compensated. The temperature range will vary only around 10-20C. >>>4. Relatively low noise(not that big of an issue since caps can take care of the big problems). >>> >>> >>>Vin ranges from 0 to 5V and Vcc ranges from about 50V to 500V. >> >>Your requirements make no sense since Vcc varies over 10:1 range how could the minimum Vcc-5Rc/Re not vary as well. If you get Vout,min=0V with Vcc=50V then Vout,min will be 450V for Vcc=50V. > > > > Huh? You are the one not making any sense. The 5V is a programming voltage and has nothing to do with the output voltage.
sure it makes sense. You are not specifying exactly what you really mean by Vcc 50..500v? and how you want the output to be represented from your 0..5 volt reference? Are you looking for a 0..50 volt out regardless of Vcc or, are you looking to use what ever is sitting at the Vcc as a ratio output for the 0..5 volts input. In other words, the 0..5 volts could be viewed as a percentage of VCC, what ever is sitting there or, simply a 10:1 gain and ignore any voltage above 50 volts on the vcc ? So basically, if we had 500V sitting at the Vcc, we could get 0..500V using 0..5 volt in or, 0..50 volts only with 0..5 in.. Do you see where the confusing is really coming from now? Jamie
On Monday, March 12, 2012 7:45:55 PM UTC-4, Archival wrote:
> On Monday, March 12, 2012 6:35:04 PM UTC-5, Archival wrote: > > On Monday, March 12, 2012 8:38:25 AM UTC-5, bloggs.fred...@gmail.com wr=
ote:
> > > On Sunday, March 11, 2012 5:32:01 PM UTC-4, Archival wrote: > > > > I need a simple(=3D cheap) relatively stable(temp and noise) dc amp=
lifier that can take a voltage from 0 to 5V and output from around 0V(not c= ritical) to near Vcc(but arbitrary) somewhat linearly.
> > > >=20 > > > >=20 > > > > I'm thinking a simple bjt amplifier with temp compensation will wor=
k decently but the issue is linearity and range.
> > > >=20 > > > > e.g., the output voltage of an ideal CE amplifier is Vout =3D Vcc -=
Rc/Re*Vin
> > > >=20 > > > > Of course when Vin =3D 0 volts, Vout =3D Vcc. When Vin is 5V we get=
Vout =3D Vcc - 5Rc/Re but we would like 0V or some low fixed voltage inste= ad.=20
> > > >=20 > > > > Adding temperature compensation makes things worse since it effects=
our upper range(which is more crucial than the lower range) since Vin cann= ot swing down to 0V(I'm assuming the simple diode compensation scheme gener= ally used).
> > > >=20 > > > > In any case the requirements are > > > >=20 > > > > 1. Relatively cheap and easy to built(e.g., a few discrete componen=
ts)/
> > > > 2. Amplifies a voltage from [0, Vin_max] to approximately [0, Vcc] =
with the upper range being more important. Vcc is somewhat arbitrary =3D=3D=
> May change after the design of the circuit =3D=3D> no component values ca=
n depend on Vcc to achieve specs. (Obviously one can assume that Vcc is wit= hin all the maximum voltage ratings of the components)
> > > > 3. Temperature stable/compensated. The temperature range will vary =
only around 10-20C.
> > > > 4. Relatively low noise(not that big of an issue since caps can tak=
e care of the big problems).
> > > >=20 > > > >=20 > > > > Vin ranges from 0 to 5V and Vcc ranges from about 50V to 500V. > > >=20 > > > Your requirements make no sense since Vcc varies over 10:1 range how =
could the minimum Vcc-5Rc/Re not vary as well. If you get Vout,min=3D0V wit= h Vcc=3D50V then Vout,min will be 450V for Vcc=3D50V.
> >=20 > >=20 > > Huh? You are the one not making any sense. The 5V is a programming volt=
age and has nothing to do with the output voltage.
>=20 > Well, I see. You seem to be thinking that Vout can change during operatio=
n and this is not the case.
>=20 > Vout is arbitrary but not changing. What this means is that is not specif=
ically specified. In the real circuit it might be 234.32V but will not chan= ge in that circuit. What I do know is that it will be between 50V and 450V.= If I can design a circuit that works over the whole range then I won't hav= e to worry about changing resistors.
>=20 > A simple example, is, say, the voltage is user specified. The user and ch=
ange it to be between 50V and 450V. I would like to design the circuit so i= t behaves the same regardless without having to change resistors to make it= so. See figure 94 on page 43 of http://www.ti.com/lit/an/snoa653/snoa653.pdf=20 A $0.44 part from Mouser http://www.mouser.com/ProductDetail/Texas-Instrume= nts/LM3900N/?qs=3DsGAEpiMZZMtYFXwiBRPs0zX5IIR%252b49bN almost 11,000 in sto= ck, can ship immediately, available in unit quantities- a very easy amplifi= er to work with. Watch the voltage ratings on your resistors, when in doubt= keep the actual less than half their maximum working voltage by using seve= ral in series to get the value you need, and consider the possible effects = of component failure putting HV back into your low voltage circuits- use ze= ner or TVS clamps, fuses, or metal oxide resistors as necessary to limit th= e damage.
On Mar 12, 11:45=A0pm, Archival <archival...@gmail.com> wrote:
> On Monday, March 12, 2012 6:35:04 PM UTC-5, Archival wrote: > > On Monday, March 12, 2012 8:38:25 AM UTC-5, bloggs.fred...@gmail.com wr=
ote:
> > > On Sunday, March 11, 2012 5:32:01 PM UTC-4, Archival wrote: > > > > I need a simple(=3D cheap) relatively stable(temp and noise) dc amp=
lifier that can take a voltage from 0 to 5V and output from around 0V(not c= ritical) to near Vcc(but arbitrary) somewhat linearly.
> > > > > I'm thinking a simple bjt amplifier with temp compensation will wor=
k decently but the issue is linearity and range.
> > > > > e.g., the output voltage of an ideal CE amplifier is Vout =3D Vcc -=
Rc/Re*Vin
> > > > > Of course when Vin =3D 0 volts, Vout =3D Vcc. When Vin is 5V we get=
Vout =3D Vcc - 5Rc/Re but we would like 0V or some low fixed voltage inste= ad.
> > > > > Adding temperature compensation makes things worse since it effects=
our upper range(which is more crucial than the lower range) since Vin cann= ot swing down to 0V(I'm assuming the simple diode compensation scheme gener= ally used).
> > > > > In any case the requirements are > > > > > 1. Relatively cheap and easy to built(e.g., a few discrete componen=
ts)/
> > > > 2. Amplifies a voltage from [0, Vin_max] to approximately [0, Vcc] =
with the upper range being more important. Vcc is somewhat arbitrary =3D=3D=
> May change after the design of the circuit =3D=3D> no component values ca=
n depend on Vcc to achieve specs. (Obviously one can assume that Vcc is wit= hin all the maximum voltage ratings of the components)
> > > > 3. Temperature stable/compensated. The temperature range will vary =
only around 10-20C.
> > > > 4. Relatively low noise(not that big of an issue since caps can tak=
e care of the big problems).
> > > > > Vin ranges from 0 to 5V and Vcc ranges from about 50V to 500V. > > > > Your requirements make no sense since Vcc varies over 10:1 range how =
could the minimum Vcc-5Rc/Re not vary as well. If you get Vout,min=3D0V wit= h Vcc=3D50V then Vout,min will be 450V for Vcc=3D50V.
> > > Huh? You are the one not making any sense. The 5V is a programming volt=
age and has nothing to do with the output voltage.
> > Well, I see. You seem to be thinking that Vout can change during operatio=
n and this is not the case.
> > Vout is arbitrary but not changing. What this means is that is not specif=
ically specified. In the real circuit it might be 234.32V but will not chan= ge in that circuit. What I do know is that it will be between 50V and 450V.= If I can design a circuit that works over the whole range then I won't hav= e to worry about changing resistors.
> > A simple example, is, say, the voltage is user specified. The user and ch=
ange it to be between 50V and 450V. I would like to design the circuit so i= t behaves the same regardless without having to change resistors to make it= so. Its not hard to use an opamp on reduced voltage to feed an HT discrete output stage, and divide down the output for opamp feedback. Perhaps you could clarify where the problem is with that. NT
NT <meow2222@care2.com> wrote:
> On Mar 12, 11:45 pm, Archival <archival...@gmail.com> wrote: >> On Monday, March 12, 2012 6:35:04 PM UTC-5, Archival wrote: >>> On Monday, March 12, 2012 8:38:25 AM UTC-5, bloggs.fred...@gmail.com wrote: >>>> On Sunday, March 11, 2012 5:32:01 PM UTC-4, Archival wrote: >>>>> I need a simple(= cheap) relatively stable(temp and noise) dc >>>>> amplifier that can take a voltage from 0 to 5V and output from around >>>>> 0V(not critical) to near Vcc(but arbitrary) somewhat linearly. >> >>>>> I'm thinking a simple bjt amplifier with temp compensation will work >>>>> decently but the issue is linearity and range. >> >>>>> e.g., the output voltage of an ideal CE amplifier is Vout = Vcc - Rc/Re*Vin >> >>>>> Of course when Vin = 0 volts, Vout = Vcc. When Vin is 5V we get Vout >>>>> = Vcc - 5Rc/Re but we would like 0V or some low fixed voltage instead. >> >>>>> Adding temperature compensation makes things worse since it effects >>>>> our upper range(which is more crucial than the lower range) since Vin >>>>> cannot swing down to 0V(I'm assuming the simple diode compensation >>>>> scheme generally used). >> >>>>> In any case the requirements are >> >>>>> 1. Relatively cheap and easy to built(e.g., a few discrete components)/ >>>>> 2. Amplifies a voltage from [0, Vin_max] to approximately [0, Vcc] >>>>> with the upper range being more important. Vcc is somewhat arbitrary >>>>> ==> May change after the design of the circuit ==> no component >>>>> values can depend on Vcc to achieve specs. (Obviously one can assume >>>>> that Vcc is within all the maximum voltage ratings of the components) >>>>> 3. Temperature stable/compensated. The temperature range will vary only around 10-20C. >>>>> 4. Relatively low noise(not that big of an issue since caps can take >>>>> care of the big problems). >> >>>>> Vin ranges from 0 to 5V and Vcc ranges from about 50V to 500V. >> >>>> Your requirements make no sense since Vcc varies over 10:1 range how >>>> could the minimum Vcc-5Rc/Re not vary as well. If you get Vout,min=0V >>>> with VccPV then Vout,min will be 450V for VccPV. >> >>> Huh? You are the one not making any sense. The 5V is a programming >>> voltage and has nothing to do with the output voltage. >> >> Well, I see. You seem to be thinking that Vout can change during >> operation and this is not the case. >> >> Vout is arbitrary but not changing. What this means is that is not >> specifically specified. In the real circuit it might be 234.32V but will >> not change in that circuit. What I do know is that it will be between >> 50V and 450V. If I can design a circuit that works over the whole range >> then I won't have to worry about changing resistors. >> >> A simple example, is, say, the voltage is user specified. The user and >> change it to be between 50V and 450V. I would like to design the circuit >> so it behaves the same regardless without having to change resistors to make it so. > > Its not hard to use an opamp on reduced voltage to feed an HT discrete > output stage, and divide down the output for opamp feedback. Perhaps > you could clarify where the problem is with that. > > > NT
The required gain is a function of Vcc. That's where all the ugly is.
On Mon, 12 Mar 2012 16:47:10 -0700 (PDT), Archival =
<archival998@gmail.com>
wrote:

>On Monday, March 12, 2012 7:30:23 AM UTC-5, John Devereux wrote: >> Archival <archival998@gmail.com> writes: >>=20 >> > I need a simple(=3D cheap) relatively stable(temp and noise) dc =
amplifier that can take a voltage from 0 to 5V and output from around = 0V(not critical) to near Vcc(but arbitrary) somewhat linearly.
>> > >> > >> > I'm thinking a simple bjt amplifier with temp compensation will work=
decently but the issue is linearity and range.
>> > >> > e.g., the output voltage of an ideal CE amplifier is Vout =3D Vcc - =
Rc/Re*Vin
>> > >> > Of course when Vin =3D 0 volts, Vout =3D Vcc. When Vin is 5V we get =
Vout =3D Vcc - 5Rc/Re but we would like 0V or some low fixed voltage = instead.=20
>> > >> > Adding temperature compensation makes things worse since it effects =
our upper range(which is more crucial than the lower range) since Vin = cannot swing down to 0V(I'm assuming the simple diode compensation scheme= generally used).
>> > >> > In any case the requirements are >> > >> > 1. Relatively cheap and easy to built(e.g., a few discrete =
components)/
>> > 2. Amplifies a voltage from [0, Vin_max] to approximately [0, Vcc] =
with the upper range being more important. Vcc is somewhat arbitrary = =3D=3D> May change after the design of the circuit =3D=3D> no component = values can depend on Vcc to achieve specs. (Obviously one can assume that= Vcc is within all the maximum voltage ratings of the components)
>> > 3. Temperature stable/compensated. The temperature range will vary =
only around 10-20C.
>> > 4. Relatively low noise(not that big of an issue since caps can take=
care of the big problems).
>> > >> > >> > Vin ranges from 0 to 5V and Vcc ranges from about 50V to 500V. >>=20 >>=20 >> I assume you want discrete because of the high Vcc? >>=20 >> I think you will need an opamp as a "front-end" since it will be hard =
to
>> get good accuracy from discretes. Transistors on opamps are all =
matched.
>>=20 >> So an opamp followed by a discrete booster stage with overall DC >> feedback to keep it accurate. >>=20 >>=20 >> --=20 >>=20 >> John Devereux > > > >On Monday, March 12, 2012 7:30:23 AM UTC-5, John Devereux wrote: >> Archival <archival998@gmail.com> writes: >>=20 >> > I need a simple(=3D cheap) relatively stable(temp and noise) dc =
amplifier that can take a voltage from 0 to 5V and output from around = 0V(not critical) to near Vcc(but arbitrary) somewhat linearly.
>> > >> > >> > I'm thinking a simple bjt amplifier with temp compensation will work=
decently but the issue is linearity and range.
>> > >> > e.g., the output voltage of an ideal CE amplifier is Vout =3D Vcc - =
Rc/Re*Vin
>> > >> > Of course when Vin =3D 0 volts, Vout =3D Vcc. When Vin is 5V we get =
Vout =3D Vcc - 5Rc/Re but we would like 0V or some low fixed voltage = instead.=20
>> > >> > Adding temperature compensation makes things worse since it effects =
our upper range(which is more crucial than the lower range) since Vin = cannot swing down to 0V(I'm assuming the simple diode compensation scheme= generally used).
>> > >> > In any case the requirements are >> > >> > 1. Relatively cheap and easy to built(e.g., a few discrete =
components)/
>> > 2. Amplifies a voltage from [0, Vin_max] to approximately [0, Vcc] =
with the upper range being more important. Vcc is somewhat arbitrary = =3D=3D> May change after the design of the circuit =3D=3D> no component = values can depend on Vcc to achieve specs. (Obviously one can assume that= Vcc is within all the maximum voltage ratings of the components)
>> > 3. Temperature stable/compensated. The temperature range will vary =
only around 10-20C.
>> > 4. Relatively low noise(not that big of an issue since caps can take=
care of the big problems).
>> > >> > >> > Vin ranges from 0 to 5V and Vcc ranges from about 50V to 500V. >>=20 >>=20 >> I assume you want discrete because of the high Vcc? >>=20 >> I think you will need an opamp as a "front-end" since it will be hard =
to
>> get good accuracy from discretes. Transistors on opamps are all =
matched.
>>=20 >> So an opamp followed by a discrete booster stage with overall DC >> feedback to keep it accurate. >>=20 > >Yes, It is not difficult to find high voltage discrete components. I was=
hoping someone would have a simple way to do this. Accuracy is not the = main issue here.
> >As long as the thing doesn't drift too much with temperature, oscillate,=
or is too noisy then it shouldn't be a problem. As long as the problems = can be reduced through capacitance I don't see any issue.
> >I see this as as simple DC amplifier and it should be able to be done =
like any others. Using a CE driving a CC does the job theoretically = excluding the range mapping issues.
> >The issue with using op amps is making them work well in the HV system. =
Seems like it may not be the safest thing to do and may add extra cost = and complexity when a simple BJT might work. I've seen a lot of tricks = with BJT amplifiers and I'm hoping there maybe one for my case. Oh hell, it can be done easily enough and safely enough. It is just that partial opamp solutions are usually easier, cheaper and more effective done correctly. ?-)
On Mon, 12 Mar 2012 16:45:55 -0700 (PDT), Archival =
<archival998@gmail.com>
wrote:

>On Monday, March 12, 2012 6:35:04 PM UTC-5, Archival wrote: >> On Monday, March 12, 2012 8:38:25 AM UTC-5, bloggs.fred...@gmail.com =
wrote:
>> > On Sunday, March 11, 2012 5:32:01 PM UTC-4, Archival wrote: >> > > I need a simple(=3D cheap) relatively stable(temp and noise) dc =
amplifier that can take a voltage from 0 to 5V and output from around = 0V(not critical) to near Vcc(but arbitrary) somewhat linearly.
>> > >=20 >> > >=20 >> > > I'm thinking a simple bjt amplifier with temp compensation will =
work decently but the issue is linearity and range.
>> > >=20 >> > > e.g., the output voltage of an ideal CE amplifier is Vout =3D Vcc =
- Rc/Re*Vin
>> > >=20 >> > > Of course when Vin =3D 0 volts, Vout =3D Vcc. When Vin is 5V we =
get Vout =3D Vcc - 5Rc/Re but we would like 0V or some low fixed voltage = instead.=20
>> > >=20 >> > > Adding temperature compensation makes things worse since it =
effects our upper range(which is more crucial than the lower range) since= Vin cannot swing down to 0V(I'm assuming the simple diode compensation = scheme generally used).
>> > >=20 >> > > In any case the requirements are >> > >=20 >> > > 1. Relatively cheap and easy to built(e.g., a few discrete =
components)/
>> > > 2. Amplifies a voltage from [0, Vin_max] to approximately [0, Vcc]=
with the upper range being more important. Vcc is somewhat arbitrary = =3D=3D> May change after the design of the circuit =3D=3D> no component = values can depend on Vcc to achieve specs. (Obviously one can assume that= Vcc is within all the maximum voltage ratings of the components)
>> > > 3. Temperature stable/compensated. The temperature range will vary=
only around 10-20C.
>> > > 4. Relatively low noise(not that big of an issue since caps can =
take care of the big problems).
>> > >=20 >> > >=20 >> > > Vin ranges from 0 to 5V and Vcc ranges from about 50V to 500V. >> >=20 >> > Your requirements make no sense since Vcc varies over 10:1 range how=
could the minimum Vcc-5Rc/Re not vary as well. If you get Vout,min=3D0V = with Vcc=3D50V then Vout,min will be 450V for Vcc=3D50V.
>>=20 >>=20 >> Huh? You are the one not making any sense. The 5V is a programming =
voltage and has nothing to do with the output voltage.
> >Well, I see. You seem to be thinking that Vout can change during =
operation and this is not the case.
> >Vout is arbitrary but not changing. What this means is that is not =
specifically specified. In the real circuit it might be 234.32V but will = not change in that circuit. What I do know is that it will be between 50V= and 450V. If I can design a circuit that works over the whole range then= I won't have to worry about changing resistors.
> >A simple example, is, say, the voltage is user specified. The user and =
change it to be between 50V and 450V. I would like to design the circuit = so it behaves the same regardless without having to change resistors to = make it so. Bloody hell. What is the output current and is there any accuracy specification? How do expect to design, let alone build, anything so piss poorly specified /:-((
On 2012-03-11, Archival <archival998@gmail.com> wrote:
> I need a simple(= cheap) relatively stable(temp and noise) dc > amplifier that can take a voltage from 0 to 5V and output from around > 0V(not critical) to near Vcc(but arbitrary) somewhat linearly.
> Vin ranges from 0 to 5V and Vcc ranges from about 50V to 500V.
use the 0-5V signal to program a rail-to-rail PWM output. filter as neccessary. -- &#9858;&#9859; 100% natural --- Posted via news://freenews.netfront.net/ - Complaints to news@netfront.net ---
On 2012-03-13, bloggs.fredbloggs.fred@gmail.com <bloggs.fredbloggs.fred@gmail.com> wrote:

> See figure 94 on page 43 of http://www.ti.com/lit/an/snoa653/snoa653.pdf
but that has fixed gain. OP wants gain proportional to VCC. Figure 3 has the current mirror drawn in a way I've not seen before. -- &#9858;&#9859; 100% natural --- Posted via news://freenews.netfront.net/ - Complaints to news@netfront.net ---
josephkk wrote:

> On Mon, 12 Mar 2012 16:45:55 -0700 (PDT), Archival <archival998@gmail.com> > wrote: > > >>On Monday, March 12, 2012 6:35:04 PM UTC-5, Archival wrote: >> >>>On Monday, March 12, 2012 8:38:25 AM UTC-5, bloggs.fred...@gmail.com wrote: >>> >>>>On Sunday, March 11, 2012 5:32:01 PM UTC-4, Archival wrote: >>>> >>>>>I need a simple(= cheap) relatively stable(temp and noise) dc amplifier that can take a voltage from 0 to 5V and output from around 0V(not critical) to near Vcc(but arbitrary) somewhat linearly. >>>>> >>>>> >>>>>I'm thinking a simple bjt amplifier with temp compensation will work decently but the issue is linearity and range. >>>>> >>>>>e.g., the output voltage of an ideal CE amplifier is Vout = Vcc - Rc/Re*Vin >>>>> >>>>>Of course when Vin = 0 volts, Vout = Vcc. When Vin is 5V we get Vout = Vcc - 5Rc/Re but we would like 0V or some low fixed voltage instead. >>>>> >>>>>Adding temperature compensation makes things worse since it effects our upper range(which is more crucial than the lower range) since Vin cannot swing down to 0V(I'm assuming the simple diode compensation scheme generally used). >>>>> >>>>>In any case the requirements are >>>>> >>>>>1. Relatively cheap and easy to built(e.g., a few discrete components)/ >>>>>2. Amplifies a voltage from [0, Vin_max] to approximately [0, Vcc] with the upper range being more important. Vcc is somewhat arbitrary ==> May change after the design of the circuit ==> no component values can depend on Vcc to achieve specs. (Obviously one can assume that Vcc is within all the maximum voltage ratings of the components) >>>>>3. Temperature stable/compensated. The temperature range will vary only around 10-20C. >>>>>4. Relatively low noise(not that big of an issue since caps can take care of the big problems). >>>>> >>>>> >>>>>Vin ranges from 0 to 5V and Vcc ranges from about 50V to 500V. >>>> >>>>Your requirements make no sense since Vcc varies over 10:1 range how could the minimum Vcc-5Rc/Re not vary as well. If you get Vout,min=0V with Vcc=50V then Vout,min will be 450V for Vcc=50V. >>> >>> >>>Huh? You are the one not making any sense. The 5V is a programming voltage and has nothing to do with the output voltage. >> >>Well, I see. You seem to be thinking that Vout can change during operation and this is not the case. >> >>Vout is arbitrary but not changing. What this means is that is not specifically specified. In the real circuit it might be 234.32V but will not change in that circuit. What I do know is that it will be between 50V and 450V. If I can design a circuit that works over the whole range then I won't have to worry about changing resistors. >> >>A simple example, is, say, the voltage is user specified. The user and change it to be between 50V and 450V. I would like to design the circuit so it behaves the same regardless without having to change resistors to make it so. > > > Bloody hell. What is the output current and is there any accuracy > specification? > How do expect to design, let alone build, anything so piss poorly > specified > > /:-((
it's simple actually.. He wants a selectable gain control. If the VCC is to change, then the gain control can be governed via the VCC level. Simple actually. A 50V minimum fixed reference against the VCC to generate a gain reference.. The 0..5V in the input to choice the level of that. Yes, a simple OP-Amp circuit with a gain pot on the front end would work find. The 0..5Volts would simply be the range of the set gain.. And if there is not enough VCC on the rails to meet the output as required, then I guess one could even design in a fail safe error logic bit output :) Oh well, sounds like something I can whip together in my sleep.. ! Jamie
Jamie <jamie_ka1lpa_not_valid_after_ka1lpa_@charter.net> writes:

> josephkk wrote: > >> On Mon, 12 Mar 2012 16:45:55 -0700 (PDT), Archival <archival998@gmail.com> >> wrote: >> >> >>>On Monday, March 12, 2012 6:35:04 PM UTC-5, Archival wrote: >>> >>>>On Monday, March 12, 2012 8:38:25 AM UTC-5, bloggs.fred...@gmail.com wrote: >>>> >>>>>On Sunday, March 11, 2012 5:32:01 PM UTC-4, Archival wrote: >>>>> >>>>>>I need a simple(= cheap) relatively stable(temp and noise) dc amplifier that can take a voltage from 0 to 5V and output from around 0V(not critical) to near Vcc(but arbitrary) somewhat linearly. >>>>>> >>>>>> >>>>>>I'm thinking a simple bjt amplifier with temp compensation will work decently but the issue is linearity and range. >>>>>> >>>>>>e.g., the output voltage of an ideal CE amplifier is Vout = Vcc - Rc/Re*Vin >>>>>> >>>>>> Of course when Vin = 0 volts, Vout = Vcc. When Vin is 5V we get >>>>>> Vout = Vcc - 5Rc/Re but we would like 0V or some low fixed >>>>>> voltage instead. >>>>>> >>>>>>Adding temperature compensation makes things worse since it effects our upper range(which is more crucial than the lower range) since Vin cannot swing down to 0V(I'm assuming the simple diode compensation scheme generally used). >>>>>> >>>>>>In any case the requirements are >>>>>> >>>>>>1. Relatively cheap and easy to built(e.g., a few discrete components)/ >>>>>>2. Amplifies a voltage from [0, Vin_max] to approximately [0, Vcc] with the upper range being more important. Vcc is somewhat arbitrary ==> May change after the design of the circuit ==> no component values can depend on Vcc to achieve specs. (Obviously one can assume that Vcc is within all the maximum voltage ratings of the components) >>>>>>3. Temperature stable/compensated. The temperature range will vary only around 10-20C. >>>>>>4. Relatively low noise(not that big of an issue since caps can take care of the big problems). >>>>>> >>>>>> >>>>>>Vin ranges from 0 to 5V and Vcc ranges from about 50V to 500V. >>>>> >>>>>Your requirements make no sense since Vcc varies over 10:1 range how could the minimum Vcc-5Rc/Re not vary as well. If you get Vout,min=0V with Vcc=50V then Vout,min will be 450V for Vcc=50V. >>>> >>>> >>>>Huh? You are the one not making any sense. The 5V is a programming voltage and has nothing to do with the output voltage. >>> >>>Well, I see. You seem to be thinking that Vout can change during operation and this is not the case. >>> >>>Vout is arbitrary but not changing. What this means is that is not specifically specified. In the real circuit it might be 234.32V but will not change in that circuit. What I do know is that it will be between 50V and 450V. If I can design a circuit that works over the whole range then I won't have to worry about changing resistors. >>> >>>A simple example, is, say, the voltage is user specified. The user and change it to be between 50V and 450V. I would like to design the circuit so it behaves the same regardless without having to change resistors to make it so. >> >> >> Bloody hell. What is the output current and is there any accuracy >> specification? >> How do expect to design, let alone build, anything so piss poorly >> specified >> >> /:-(( > it's simple actually.. > > He wants a selectable gain control. > > If the VCC is to change, then the gain control can be governed via > the VCC level. > Simple actually. A 50V minimum fixed reference against the VCC to > generate a gain reference.. > > The 0..5V in the input to choice the level of that. > > Yes, a simple OP-Amp circuit with a gain pot on the front end > would work find. The 0..5Volts would simply be the range of the set > gain.. > > And if there is not enough VCC on the rails to meet the output as > required, then I guess one could even design in a fail safe error > logic bit output :) > > > Oh well, sounds like something I can whip together in my sleep.. !
Well he said he did not want to "change resistors" so I would take that to include not wanted to adjust a pot either! Also he does not seemed to have mentioned the output current, a critical little detail at 500V... -- John Devereux