# Stable simple dc amplifiers

Started by March 11, 2012
```I need a simple(=3D cheap) relatively stable(temp and noise) dc amplifier t=
hat 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 decent=
ly but the issue is linearity and range.

e.g., the output voltage of an ideal CE amplifier is Vout =3D Vcc - Rc/Re*V=
in

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 upp=
er 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 use=
d).

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 ch=
ange 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 aro=
und 10-20C.
4. Relatively low noise(not that big of an issue since caps can take care o=
f the big problems).

Vin ranges from 0 to 5V and Vcc ranges from about 50V to 500V.

```
```On Sun, 11 Mar 2012 14:32:01 -0700 (PDT), 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.
>
>
>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.
>
>

Why not an opamp?

--

John Larkin, President       Highland Technology Inc
www.highlandtechnology.com   jlarkin at highlandtechnology dot com

Precision electronic instrumentation
Picosecond-resolution Digital Delay and Pulse generators
Custom timing and laser controllers
Photonics and fiberoptic TTL data links
VME  analog, thermocouple, LVDT, synchro, tachometer
Multichannel arbitrary waveform generators
```
```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.
>
>
>
There are a lot of opamps that can fill the bill..put a FET at the
output and use some feedback; gain of 100 good for 0-5V in, 0-500V out.

```
```Archival <archival998@gmail.com> writes:

> 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.

I assume you want discrete because of the high Vcc?

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.

So an opamp followed by a discrete booster stage with overall DC
feedback to keep it accurate.

--

John Devereux
```
```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 critica=
l) to near Vcc(but arbitrary) somewhat linearly.
>=20
>=20
> I'm thinking a simple bjt amplifier with temp compensation will work dece=
ntly 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 u=
pper range(which is more crucial than the lower range) since Vin cannot swi=
ng down to 0V(I'm assuming the simple diode compensation scheme generally u=
sed).
>=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 t=
he 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 depe=
nd on Vcc to achieve specs. (Obviously one can assume that Vcc is within al=
l the maximum voltage ratings of the components)
> 3. Temperature stable/compensated. The temperature range will vary only a=
round 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.

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.
```
```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 amplifi=
er that can take a voltage from 0 to 5V and output from around 0V(not criti=
cal) to near Vcc(but arbitrary) somewhat linearly.
> >=20
> >=20
> > I'm thinking a simple bjt amplifier with temp compensation will work de=
cently 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 Vou=
t =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 s=
wing 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> Ma=
y change after the design of the circuit =3D=3D> no component values can de=
pend 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 ca=
re 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 coul=
d the minimum Vcc-5Rc/Re not vary as well. If you get Vout,min=3D0V with Vc=
c=3D50V then Vout,min will be 450V for Vcc=3D50V.

Huh? You are the one not making any sense. The 5V is a programming voltage =
and has nothing to do with the output voltage.
```
```On Monday, March 12, 2012 7:35:04 PM UTC-4, Archival wrote:
> On Monday, March 12, 2012 8:38:25 AM UTC-5, bloggs.fred...@gmail.com wrot=
e:
> > 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 ampli=
fier that can take a voltage from 0 to 5V and output from around 0V(not cri=
tical) 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 - R=
c/Re*Vin
> > >=20
> > > Of course when Vin =3D 0 volts, Vout =3D Vcc. When Vin is 5V we get V=
out =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 o=
ur 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 general=
ly 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] wi=
th 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 withi=
n all the maximum voltage ratings of the components)
> > > 3. Temperature stable/compensated. The temperature range will vary on=
ly 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 co=
uld 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 voltag=
e and has nothing to do with the output voltage.

Huh? Was it you or someone else who said "I need a simple(=3D cheap) relati=
vely 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) somew=
hat linearly." That sounds like you want a transfer function of:
Vout=3DVcc x (1-Vin/5)
which cannot be achieved with a CE amplifier with output of the form:
Vout=3DVcc-VinRc/Re for variable Vcc over 10:1 range (50 to 500 volts) and=
everything else fixed.=20
You need a different kind of amplifier, and you need to tell us the loading=
on the amplifier.=20

```
```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 wrot=
e:
> > 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 ampli=
fier that can take a voltage from 0 to 5V and output from around 0V(not cri=
tical) 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 - R=
c/Re*Vin
> > >=20
> > > Of course when Vin =3D 0 volts, Vout =3D Vcc. When Vin is 5V we get V=
out =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 o=
ur 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 general=
ly 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] wi=
th 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 withi=
n all the maximum voltage ratings of the components)
> > > 3. Temperature stable/compensated. The temperature range will vary on=
ly 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 co=
uld 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 voltag=
e 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 specific=
ally 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. I=
f I can design a circuit that works over the whole range then I won't have =

A simple example, is, say, the voltage is user specified. The user and chan=
ge 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 s=
o.
```
```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 amplifi=
er that can take a voltage from 0 to 5V and output from around 0V(not criti=
cal) to near Vcc(but arbitrary) somewhat linearly.
> >
> >
> > I'm thinking a simple bjt amplifier with temp compensation will work de=
cently 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 Vou=
t =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 s=
wing 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> Ma=
y change after the design of the circuit =3D=3D> no component values can de=
pend 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 ca=
re 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 amplifi=
er that can take a voltage from 0 to 5V and output from around 0V(not criti=
cal) to near Vcc(but arbitrary) somewhat linearly.
> >
> >
> > I'm thinking a simple bjt amplifier with temp compensation will work de=
cently 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 Vou=
t =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 s=
wing 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> Ma=
y change after the design of the circuit =3D=3D> no component values can de=
pend 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 ca=
re 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 ho=
ping someone would have a simple way to do this. Accuracy is not the main i=
ssue 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 b=
e 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 th=
e range mapping issues.

The issue with using op amps is making them work well in the HV system. See=
ms like it may not be the safest thing to do and may add extra cost and com=
plexity when a simple BJT might work. I've seen a lot of tricks with BJT am=
plifiers and I'm hoping there maybe one for my case.
```
```On Monday, March 12, 2012 12:42:02 AM UTC-5, Robert Baer wrote:
> Archival wrote:
> > I need a simple(=3D cheap) relatively stable(temp and noise) dc amplifi=
er that can take a voltage from 0 to 5V and output from around 0V(not criti=
cal) to near Vcc(but arbitrary) somewhat linearly.
> >
> >
> > I'm thinking a simple bjt amplifier with temp compensation will work de=
cently 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 Vou=
t =3D 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 s=
wing 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>  M=
ay 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 withi=
n 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 ca=
re of the big problems).
> >
> >
> > Vin ranges from 0 to 5V and Vcc ranges from about 50V to 500V.
> >
> >
> >
>    There are a lot of opamps that can fill the bill..put a FET at the=20
> output and use some feedback; gain of 100 good for 0-5V in, 0-500V out.

What do you mean a lot of op amps? High voltage op amps? You know a few tha=
t don't cost more than a few \$?
```