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How to select transistor for oscillator ?

Started by Unknown September 11, 2018
On 09/11/2018 11:45 PM, tabbypurr@gmail.com wrote:
> On Wednesday, 12 September 2018 04:11:47 UTC+1, bitrex wrote: >> On 09/11/2018 10:16 PM, John Larkin wrote: >>> On Tue, 11 Sep 2018 13:32:35 -0400, bitrex <user@example.net> wrote: >>> >>>> On 09/11/2018 11:36 AM, John Larkin wrote: >>>>> On Mon, 10 Sep 2018 20:38:48 -0700 (PDT), dakupoto@gmail.com wrote: >>>>> >>>>>> Could some electronics guru here please help ? >>>>>> What is the best way to select a transistor >>>>>> for an oscillator ? Consider a Colpitts >>>>>> oscillator running at 5 MHz. The following are >>>>>> the analysis steps. >>>>>> 1. Select a value for the LC tank capacitor >>>>>> C1 = C2 = C >>>>>> 2. Select a load e.g., R=50 Ohm >>>>>> 3. To start oscillations, gmR > C2/C1. In this >>>>>> case gmR > 1 >>>>>> 4. Using the oscillation frequency and the LC >>>>>> tank capacitance value, the inductor value >>>>>> is obtained. >>>>>> 5. Having determined gm, find the collector >>>>>> current using gm = Ic/VT where VT = 0.025 >>>>>> 6. From the VCC value, the collector current >>>>>> limiting resistor is VCC/Ic >>>>>> 7. The base bias resistors are then obtained >>>>>> using the fact that the maximum base bias >>>>>> current is 0.1 times the collector current. >>>>>> Then the question is: how to select a transistor ? >>>>>> >>>>>> All hints/suggestions are helpful. Thanks in advance >>>>> >>>>> >>>>> My common NPN gumdrops are BCX70 and BFS17 for faster stuff. >>>>> >>>>> If you care about noise, take a look at Win's table, page 501 of AOE3. >>>>> It pains me that he selected BCX70 as the first/worse transistor in >>>>> the list. >>>>> >>>>> Actually, I rarely use bipolar transistors these days. >>>>> >>>>> >>>> >>>> Unless one really need low phase noise or distortion or very high >>>> frequency or some other specific parameter from an oscillator I think >>>> detailed mathematical analysis of low-frequency oscillators is kinda a >>>> waste of time. Yeah you can do it but the circuits are nonlinear and the >>>> results are often not particularly enlightening. This is what SPICE was >>>> invented for. >>>> >>> >>> Can Spice do useful noise analysis of oscillators? >>> >>> >> >> The "problem" such as it is, is that OP asked "what the BEST transistor" >> is and as usual the answer is "it depends." He didn't specify a noise >> requirement just a 5MHz Colpitts. so why not just a 2N3904? They work >> fine for the Colpitts at that frequency and there are many examples >> online. >> >> It's kinda a reinvent-the-wheel situation usually when you go to >> reinvent-the-wheel you a) don't really learn too much and b) end up with >> a crappy wheel. > > Asking what the best thing is is always the wrong approach. But the OP said 'What is the best way to select a transistor for an oscillator ?' > > > NT >
The best part about engineers is that if I'm ever stranded with a dead battery in the middle of nowhere all I need to do is say "So I'll just connect up the red to the black and the...." out loud and within 10 sec one will be on scene to a) call me out on the error b) do the work for me /shrug
On 09/11/2018 11:46 PM, Clifford Heath wrote:
> On 12/09/18 13:11, bitrex wrote: >> On 09/11/2018 10:16 PM, John Larkin wrote: >>> On Tue, 11 Sep 2018 13:32:35 -0400, bitrex <user@example.net> wrote: >>> >>>> On 09/11/2018 11:36 AM, John Larkin wrote: >>>>> On Mon, 10 Sep 2018 20:38:48 -0700 (PDT), dakupoto@gmail.com wrote: >>>>> >>>>>> Could some electronics guru here please help ? >>>>>> What is the best way to select a transistor >>>>>> for an oscillator ? Consider a Colpitts >>>>>> oscillator running at 5 MHz. The following are >>>>>> the analysis steps. >>>>>> 1. Select a value for the LC tank capacitor >>>>>> &nbsp;&nbsp;&nbsp; C1 = C2 = C >>>>>> 2. Select a load e.g., R=50 Ohm >>>>>> 3. To start oscillations, gmR > C2/C1. In this >>>>>> &nbsp;&nbsp;&nbsp; case gmR > 1 >>>>>> 4. Using the oscillation frequency and the LC >>>>>> &nbsp;&nbsp;&nbsp; tank capacitance value, the inductor value >>>>>> &nbsp;&nbsp;&nbsp; is obtained. >>>>>> 5. Having determined gm, find the collector >>>>>> &nbsp;&nbsp;&nbsp; current using gm = Ic/VT where VT = 0.025 >>>>>> 6. From the VCC value, the collector current >>>>>> &nbsp;&nbsp;&nbsp; limiting resistor is VCC/Ic >>>>>> 7. The base bias resistors are then obtained >>>>>> &nbsp;&nbsp;&nbsp; using the fact that the maximum base bias >>>>>> &nbsp;&nbsp;&nbsp; current is 0.1 times the collector current. >>>>>> Then the question is: how to select a transistor ? >>>>>> >>>>>> All hints/suggestions are helpful. Thanks in advance >>>>> >>>>> >>>>> My common NPN gumdrops are BCX70 and BFS17 for faster stuff. >>>>> >>>>> If you care about noise, take a look at Win's table, page 501 of AOE3. >>>>> It pains me that he selected BCX70 as the first/worse transistor in >>>>> the list. >>>>> >>>>> Actually, I rarely use bipolar transistors these days. >>>>> >>>>> >>>> >>>> Unless one really need low phase noise or distortion or very high >>>> frequency or some other specific parameter from an oscillator I think >>>> detailed mathematical analysis of low-frequency oscillators is kinda a >>>> waste of time. Yeah you can do it but the circuits are nonlinear and >>>> the >>>> results are often not particularly enlightening. This is what SPICE was >>>> invented for. >>>> >>> >>> Can Spice do useful noise analysis of oscillators? >>> >>> >> >> The "problem" such as it is, is that OP asked "what the BEST transistor" > > No he didn't. He asked "what is the best way to select a transistor".
OK what is it?
On 09/12/2018 12:25 AM, bitrex wrote:
> On 09/11/2018 11:46 PM, Clifford Heath wrote: >> On 12/09/18 13:11, bitrex wrote: >>> On 09/11/2018 10:16 PM, John Larkin wrote: >>>> On Tue, 11 Sep 2018 13:32:35 -0400, bitrex <user@example.net> wrote: >>>> >>>>> On 09/11/2018 11:36 AM, John Larkin wrote: >>>>>> On Mon, 10 Sep 2018 20:38:48 -0700 (PDT), dakupoto@gmail.com wrote: >>>>>> >>>>>>> Could some electronics guru here please help ? >>>>>>> What is the best way to select a transistor >>>>>>> for an oscillator ? Consider a Colpitts >>>>>>> oscillator running at 5 MHz. The following are >>>>>>> the analysis steps. >>>>>>> 1. Select a value for the LC tank capacitor >>>>>>> &nbsp;&nbsp;&nbsp; C1 = C2 = C >>>>>>> 2. Select a load e.g., R=50 Ohm >>>>>>> 3. To start oscillations, gmR > C2/C1. In this >>>>>>> &nbsp;&nbsp;&nbsp; case gmR > 1 >>>>>>> 4. Using the oscillation frequency and the LC >>>>>>> &nbsp;&nbsp;&nbsp; tank capacitance value, the inductor value >>>>>>> &nbsp;&nbsp;&nbsp; is obtained. >>>>>>> 5. Having determined gm, find the collector >>>>>>> &nbsp;&nbsp;&nbsp; current using gm = Ic/VT where VT = 0.025 >>>>>>> 6. From the VCC value, the collector current >>>>>>> &nbsp;&nbsp;&nbsp; limiting resistor is VCC/Ic >>>>>>> 7. The base bias resistors are then obtained >>>>>>> &nbsp;&nbsp;&nbsp; using the fact that the maximum base bias >>>>>>> &nbsp;&nbsp;&nbsp; current is 0.1 times the collector current. >>>>>>> Then the question is: how to select a transistor ? >>>>>>> >>>>>>> All hints/suggestions are helpful. Thanks in advance >>>>>> >>>>>> >>>>>> My common NPN gumdrops are BCX70 and BFS17 for faster stuff. >>>>>> >>>>>> If you care about noise, take a look at Win's table, page 501 of >>>>>> AOE3. >>>>>> It pains me that he selected BCX70 as the first/worse transistor in >>>>>> the list. >>>>>> >>>>>> Actually, I rarely use bipolar transistors these days. >>>>>> >>>>>> >>>>> >>>>> Unless one really need low phase noise or distortion or very high >>>>> frequency or some other specific parameter from an oscillator I think >>>>> detailed mathematical analysis of low-frequency oscillators is kinda a >>>>> waste of time. Yeah you can do it but the circuits are nonlinear >>>>> and the >>>>> results are often not particularly enlightening. This is what SPICE >>>>> was >>>>> invented for. >>>>> >>>> >>>> Can Spice do useful noise analysis of oscillators? >>>> >>>> >>> >>> The "problem" such as it is, is that OP asked "what the BEST transistor" >> >> No he didn't. He asked "what is the best way to select a transistor". > > OK what is it?
I agree that you've understood the question better than I did, I'd like you to please take over at this point
On Wednesday, 12 September 2018 05:25:51 UTC+1, bitrex  wrote:
> On 09/11/2018 11:46 PM, Clifford Heath wrote: > > On 12/09/18 13:11, bitrex wrote: > >> On 09/11/2018 10:16 PM, John Larkin wrote: > >>> On Tue, 11 Sep 2018 13:32:35 -0400, bitrex <user@example.net> wrote: > >>> > >>>> On 09/11/2018 11:36 AM, John Larkin wrote: > >>>>> On Mon, 10 Sep 2018 20:38:48 -0700 (PDT), dakupoto@gmail.com wrote: > >>>>> > >>>>>> Could some electronics guru here please help ? > >>>>>> What is the best way to select a transistor > >>>>>> for an oscillator ? Consider a Colpitts > >>>>>> oscillator running at 5 MHz. The following are > >>>>>> the analysis steps. > >>>>>> 1. Select a value for the LC tank capacitor > >>>>>> &nbsp;&nbsp;&nbsp; C1 = C2 = C > >>>>>> 2. Select a load e.g., R=50 Ohm > >>>>>> 3. To start oscillations, gmR > C2/C1. In this > >>>>>> &nbsp;&nbsp;&nbsp; case gmR > 1 > >>>>>> 4. Using the oscillation frequency and the LC > >>>>>> &nbsp;&nbsp;&nbsp; tank capacitance value, the inductor value > >>>>>> &nbsp;&nbsp;&nbsp; is obtained. > >>>>>> 5. Having determined gm, find the collector > >>>>>> &nbsp;&nbsp;&nbsp; current using gm = Ic/VT where VT = 0.025 > >>>>>> 6. From the VCC value, the collector current > >>>>>> &nbsp;&nbsp;&nbsp; limiting resistor is VCC/Ic > >>>>>> 7. The base bias resistors are then obtained > >>>>>> &nbsp;&nbsp;&nbsp; using the fact that the maximum base bias > >>>>>> &nbsp;&nbsp;&nbsp; current is 0.1 times the collector current. > >>>>>> Then the question is: how to select a transistor ? > >>>>>> > >>>>>> All hints/suggestions are helpful. Thanks in advance > >>>>> > >>>>> > >>>>> My common NPN gumdrops are BCX70 and BFS17 for faster stuff. > >>>>> > >>>>> If you care about noise, take a look at Win's table, page 501 of AOE3. > >>>>> It pains me that he selected BCX70 as the first/worse transistor in > >>>>> the list. > >>>>> > >>>>> Actually, I rarely use bipolar transistors these days. > >>>>> > >>>>> > >>>> > >>>> Unless one really need low phase noise or distortion or very high > >>>> frequency or some other specific parameter from an oscillator I think > >>>> detailed mathematical analysis of low-frequency oscillators is kinda a > >>>> waste of time. Yeah you can do it but the circuits are nonlinear and > >>>> the > >>>> results are often not particularly enlightening. This is what SPICE was > >>>> invented for. > >>>> > >>> > >>> Can Spice do useful noise analysis of oscillators? > >>> > >>> > >> > >> The "problem" such as it is, is that OP asked "what the BEST transistor" > > > > No he didn't. He asked "what is the best way to select a transistor". > > OK what is it?
You need enough fT. In most cases you don't need anything else, so I'd go to a supplier's website, select trs with enough fT then order them by cost. Or if only building one, just get a random jellybean from the whatever tr boxes. NT
On Tuesday, September 11, 2018 at 11:20:23 AM UTC+5:30, Tim Williams wrote:
> 1. Equal capacitors?? But C = C1*C2 / (C1+C2)? > 2. Note this ignores component losses: inductor, capacitors, and transistor > too. > 3. To follow on from that, the reflected impedance of the base (assuming a > common emitter configuration) needs to be similarly high, otherwise the tank > goes thud, or insufficient signal gets to the base. > > Mind that, in addition to being a resonant tank and phase shift network, the > CLC is also an impedance matching network. The capacitor ratio gives the > voltage ratio, and therefore the impedance ratio squared, but that impedance > is only valid if the capacitor impedance itself is low enough. Which is > also to say, the tank Q is greater than the voltage ratio as well. > 4. I would start with Zo = sqrt(L/C) and Fo = 1 / (2*pi*sqrt(L*C)) instead, > where Zo is the resonant impedance, which will be Q times the equivalent > total load impedance. > 5. Yup, for a BJT of course; and for anything else, whatever it is. This is > minimum, of course, and probably 2-3 times more is a good idea. (The > average Gm drops at large signal levels, as amplitude saturates. In this > way, you could design an oscillator to continue to operate, at relatively > low amplitudes, without hitting saturation in the output (collector/etc.) > side.) > 6. Collector limiting?? Did you mean emitter? But that wouldn't use the > whole VCC, unsure. > > Anyway, a normal common emitter amplifier can be used here, setting, say: Vb > ~ 0.2 Vcc, Vc ~ 0.6 Vcc and therefore Ve = Vb - Vbe. > > That's Vc if a collector resistor is used, otherwise with a choke load > (which would be a tapped coil in a Hartley oscillator). Obviously, don't > forget to factor Rc into the total load resistance! > > Note that gm = 1 / (Ic/Vth + 1/R_E), that is, r_e + R_E in the hybrid-pi > model. R_E can be bypassed with a cap if desired. > > 7. Maximum base bias is Ic / hFE(min). Set base divider current (i.e., Vcc > / (Rb1 + Rb2)) to about 10x that, to ensure the base voltage doesn't droop > much. In practice, it may turn out that the base divider current is about > Ic/10, but this isn't a saturated switch we're making here. :) > > Finally, transistor only needs fT high enough. fT is not the definition of > Gm rolloff -- that requires knowing h_ie at fT -- but it is related. > Typically you'll have fT > 10*Fo. > > Tim > > -- > Seven Transistor Labs, LLC > Electrical Engineering Consultation and Design > Website: https://www.seventransistorlabs.com/ > > <dakupoto@gmail.com> wrote in message > news:6d89d386-eb4a-458b-b1e5-d43f8bafe714@googlegroups.com... > > Could some electronics guru here please help ? > > What is the best way to select a transistor > > for an oscillator ? Consider a Colpitts > > oscillator running at 5 MHz. The following are > > the analysis steps. > > 1. Select a value for the LC tank capacitor > > C1 = C2 = C > > 2. Select a load e.g., R=50 Ohm > > 3. To start oscillations, gmR > C2/C1. In this > > case gmR > 1 > > 4. Using the oscillation frequency and the LC > > tank capacitance value, the inductor value > > is obtained. > > 5. Having determined gm, find the collector > > current using gm = Ic/VT where VT = 0.025 > > 6. From the VCC value, the collector current > > limiting resistor is VCC/Ic > > 7. The base bias resistors are then obtained > > using the fact that the maximum base bias > > current is 0.1 times the collector current. > > Then the question is: how to select a transistor ? > > > > All hints/suggestions are helpful. Thanks in advance
I am afraid there is something peculiar about the atarting point of your calculations - step 4 outlined above. Using Fo = 1/(2*PI*sqrt(L*C)) LC = 4*PI*PI*Fo*Fo Now assuming a standard value for L, e.g., 1nH C=4*PI*PI*Fo*Fo*1.0E+9 Now if C is the equivalent capacitance, i.e., C = C1*C2/(C1 + C2), C1*C2/(C1 + C2) = 4*PI*PI*Fo*Fo*1.0E+9 If Fo = 5MHz, C1*C2/(C1 + C2) = 100.0*P*PI*1.0+12*1.0E+9 A very big number, i.e., the equivalent capacitance capacitance values is huge !! Most likely the expression for the equivalent capacitance is a bit strange.
On 2018-09-12, bitrex <user@example.net> wrote:

>>> The "problem" such as it is, is that OP asked "what the BEST transistor" >> >> No he didn't. He asked "what is the best way to select a transistor". > > OK what is it?
If you're right handed use your right hand. -- &#1578;
On 12/09/2018 06:01, dakupoto@gmail.com wrote:
> On Tuesday, September 11, 2018 at 11:20:23 AM UTC+5:30, Tim Williams wrote: >> 1. Equal capacitors?? But C = C1*C2 / (C1+C2)? >> 2. Note this ignores component losses: inductor, capacitors, and transistor >> too. >> 3. To follow on from that, the reflected impedance of the base (assuming a >> common emitter configuration) needs to be similarly high, otherwise the tank >> goes thud, or insufficient signal gets to the base. >> >> Mind that, in addition to being a resonant tank and phase shift network, the >> CLC is also an impedance matching network. The capacitor ratio gives the >> voltage ratio, and therefore the impedance ratio squared, but that impedance >> is only valid if the capacitor impedance itself is low enough. Which is >> also to say, the tank Q is greater than the voltage ratio as well. >> 4. I would start with Zo = sqrt(L/C) and Fo = 1 / (2*pi*sqrt(L*C)) instead, >> where Zo is the resonant impedance, which will be Q times the equivalent >> total load impedance. >> 5. Yup, for a BJT of course; and for anything else, whatever it is. This is >> minimum, of course, and probably 2-3 times more is a good idea. (The >> average Gm drops at large signal levels, as amplitude saturates. In this >> way, you could design an oscillator to continue to operate, at relatively >> low amplitudes, without hitting saturation in the output (collector/etc.) >> side.) >> 6. Collector limiting?? Did you mean emitter? But that wouldn't use the >> whole VCC, unsure. >> >> Anyway, a normal common emitter amplifier can be used here, setting, say: Vb >> ~ 0.2 Vcc, Vc ~ 0.6 Vcc and therefore Ve = Vb - Vbe. >> >> That's Vc if a collector resistor is used, otherwise with a choke load >> (which would be a tapped coil in a Hartley oscillator). Obviously, don't >> forget to factor Rc into the total load resistance! >> >> Note that gm = 1 / (Ic/Vth + 1/R_E), that is, r_e + R_E in the hybrid-pi >> model. R_E can be bypassed with a cap if desired. >> >> 7. Maximum base bias is Ic / hFE(min). Set base divider current (i.e., Vcc >> / (Rb1 + Rb2)) to about 10x that, to ensure the base voltage doesn't droop >> much. In practice, it may turn out that the base divider current is about >> Ic/10, but this isn't a saturated switch we're making here. :) >> >> Finally, transistor only needs fT high enough. fT is not the definition of >> Gm rolloff -- that requires knowing h_ie at fT -- but it is related. >> Typically you'll have fT > 10*Fo. >> >> Tim >> >> -- >> Seven Transistor Labs, LLC >> Electrical Engineering Consultation and Design >> Website: https://www.seventransistorlabs.com/ >> >> <dakupoto@gmail.com> wrote in message >> news:6d89d386-eb4a-458b-b1e5-d43f8bafe714@googlegroups.com... >>> Could some electronics guru here please help ? >>> What is the best way to select a transistor >>> for an oscillator ? Consider a Colpitts >>> oscillator running at 5 MHz. The following are >>> the analysis steps. >>> 1. Select a value for the LC tank capacitor >>> C1 = C2 = C >>> 2. Select a load e.g., R=50 Ohm >>> 3. To start oscillations, gmR > C2/C1. In this >>> case gmR > 1 >>> 4. Using the oscillation frequency and the LC >>> tank capacitance value, the inductor value >>> is obtained. >>> 5. Having determined gm, find the collector >>> current using gm = Ic/VT where VT = 0.025 >>> 6. From the VCC value, the collector current >>> limiting resistor is VCC/Ic >>> 7. The base bias resistors are then obtained >>> using the fact that the maximum base bias >>> current is 0.1 times the collector current. >>> Then the question is: how to select a transistor ? >>> >>> All hints/suggestions are helpful. Thanks in advance > > I am afraid there is something peculiar about the atarting point of your calculations - step 4 outlined above. Using Fo = 1/(2*PI*sqrt(L*C)) > LC = 4*PI*PI*Fo*Fo
You missed out the reciprocal (1/x)
> Now assuming a standard value for L, e.g., 1nH
1nH is far too low for 5MHz, try from 1 to 10 uH instead?
> C=4*PI*PI*Fo*Fo*1.0E+9 > Now if C is the equivalent capacitance, i.e., > C = C1*C2/(C1 + C2), > C1*C2/(C1 + C2) = 4*PI*PI*Fo*Fo*1.0E+9 > If Fo = 5MHz, > C1*C2/(C1 + C2) = 100.0*P*PI*1.0+12*1.0E+9 > A very big number, i.e., the equivalent capacitance capacitance values is huge !! > Most likely the expression for the equivalent > capacitance is a bit strange. >
piglet
John Larkin wrote
>On Tue, 11 Sep 2018 20:57:34 GMT, <698839253X6D445TD@nospam.org> >wrote: > >>John Larkin wrote >>>I have some CK722's! >>> >>>https://www.dropbox.com/s/lq8w4jsh0sn5fc0/Ck722-0A.JPG?raw=1 >> >>What is the Ft?
??
>>>Here's my oscillator: >>> >>>https://www.dropbox.com/s/19imyfg1ubh2z3c/P5_CCRO.jpg?raw=1 >>> >>>The CCROs have TCs, Qs, and initial accuracies over 10x better than an >>>LC. The higher-frequency parts have Qs in the thousands. >> >>Ceramics are not better than quartz crystals I think? > >They are available up into the GHz. As oscillators, they are a lot >more pullable than crystals.
How do you pull those?
>But most CCROs are probably used as >bandpass filters. > >http://www.mpdigest.com/2017/01/23/ceramic-resonator-band-pass-filter/
Yes of course, those ceramic filters, or should I say SAW filters, are everywhere, for example in TVs as IF filter. But I meant for frequency stability, for example these http://panteltje.com/pub/5_dollar_LNB_PCB_IMG_3582.GIF the greenish round things on the right are 9.75 GHz and 10.6 GHz oscillators (smallest one highest frequency), but precisely the reason I am using the latest LNB with a crystal reference and a PLL to get those same frequencies, as the crystal is much more stable, even without temperature compensation. This is needed to receive 10.4 GHz single sideband signals where a few hundred Hz at 10.4 GHz makes the speech un-intelligible. That is also where the Rubidium reference comes into play. For less precise things such as some filters ceramics are OK, those are also OK for local oscillator in LNBs for digital TV. Important for all this in my case is that you can electronically adjust frequency, say pull the crystal, so you can PLL lock to some reference. Look at the feedback coupling of those oscillator transistors to the ceramic resonators: ( O ) b c Microwave is fun !
"piglet" <erichpwagner@hotmail.com> wrote in message 
news:pnadla$pu0$1@dont-email.me...
>> I am afraid there is something peculiar about the atarting point of your >> calculations - step 4 outlined above. Using Fo = 1/(2*PI*sqrt(L*C)) >> LC = 4*PI*PI*Fo*Fo > > You missed out the reciprocal (1/x) >
Right, let alone the sanity check:
> 1nH is far too low for 5MHz, try from 1 to 10 uH instead? >
1nH is 31mohms at 5MHz, certainly less than the load resistance (50 ohms for instance), but you'll never realize a pure 1nH without more nH's in the other components, nor a Q factor probably much over 20 at that frequency, nor a transistor that can oscillate into a 0.03 ohm (reactance) * 20 Q = 0.6 ohm (resistance) load. Following the steps I outlined, say for a 50 ohm load, a voltage ratio of say 10, and a Q factor* also around 10, then Zo = 50 / 10 = 5 ohms, and L = (5 ohm) / (2*pi*(5MHz)) = 0.16uH and Ceq = 6.4nF. C2/C1 ~= 10 so C2 ~= 64nF and C1 ~= 6.4nF, but actually 10% higher for each, and give or take the exact ratio (just reverse Ceq = C1*C2 / (C1 + C2)). *The transistor's base load will be low ~kohms. The impedance, looking into the resonant tank from the base, is on the order of 1 / (2*pi*F*C2), and the resistance is lower. We should be able to ignore this for most cases; a very low power or low distortion oscillator (gm limited), may not be able to ignore this, which is why I'm not completely setting it aside. It would take further analysis to determine if a particular transistor can drive this load, and what C ratio is required for that selected transistor. After a few steps, you'd have something quite worthy of breadboarding, since it is a poor theoretician that does not verify their results in the lab. :-) Note that this is a power oscillator, or at least a very low voltage oscillator. You don't use 5 ohm tanks with small signals! If the intent is to have a little voltage into 50 ohms, not a large fraction of Vcc -- one should use another impedance matching network to derive that. A tapped or coupled inductor, or a capacitive divider (or taking the output from the base side of the tank, or the emitter circuit if unbypassed) can be used for that purpose, in which case all the circuit impedances can rise accordingly, the inductance and capacitance get much more familiar, and a general purpose transistor is suitable (say, anything fT > 50MHz, which, truly, is a hell of a lot of "general purpose" transistors). Tim -- Seven Transistor Labs, LLC Electrical Engineering Consultation and Design Website: https://www.seventransistorlabs.com/
On Tuesday, September 11, 2018 at 3:50:25 PM UTC-4, John Larkin wrote:
> On Tue, 11 Sep 2018 19:29:08 GMT, <698839253X6D445TD@nospam.org> > wrote: > > >John Larkin wrote > >>Bipolars are not unstable. But mosfets are usually easier to use. > >> > >>The last bipolar transistor circuit that I designed was in fact a > >>Colpitts oscillator. It's a 600 MHz VCO that uses a coaxial ceramic > >>resonator and a BFT25A. The time consuming bit was temperature > >>compensating it; I wound up buying a reel of custom-brewed N4700 caps. > > > >I use bipolars all the time, > >Here a 2.4 Giggle Hz oscillator: > > http://panteltje.com/pub/2.4GHz_twisted_oscillator_IMG_3629.GIF > > > >1.57 Giggle Hertz: > > http://panteltje.com/pub/GPS_jammer_board_twisted_wire_1.57GHz_oscillator_IMG_3622.GIF > > > >Think those were BFR91. > >These are both to some degree frequency controlled by the base current, > >that changes Vce and CVce and thus frequency. > > > >Anyways that was locked to the FPGA board that was locked to the Rubidium reference on the left. > > http://panteltje.com/pub/25MHz_lock_test_setup_IMG_3721.GIF > > > >6 MHz xtal oscillator: > > http://panteltje.com/pub/6MHz_xtal_oscillator.gif > > > >For lower frequencies I of course use unijunction transistors. > > > >For even higher frequencies there are 'sjips', like this I am working on: > > http://panteltje.com/pub/octagon_twin_LNB_OTLSO_inside_RT320M_PLL_IMG_6538.JPG > >The RT320M chip is a PLL locked to a 25 MHz xtal (on the right) with an output of > >390 x 25 MHz = 9.750 GHz > >or > >426 x 25 MHz = 10.650 GHz > >used as local oscillator and mixer for 10.7 GHz to 12,75 GHz reception. > >Modifying it for 10.4 GHz, > > > >Removed the crystal from the board, here testing if it still works with crystal on thin wires on the side. > >Thing is small... bad PCB, tracks fall off... > >A tunable external reference will be connected via a connector on the bottom. > > http://panteltje.com/pub/octagon_LNB_crystal_removed_IMG_6546.JPG > > > >Will attempt that locking thing with this LNB too. > > > >For even higher frequencies I have some various color lasers. > > > >:-) > > > > > >But anyways bipolars are great, the naming here in Elektor magazine was TUP and TUN for > >Transistor Universal NPN and Transistor Universal PNP, > >BC547 BC557, also known in a smaller package as > >BC847 BC857 > >have only a Ft of 100 MHz or so, good enough for most things. > > > >I also still have an old germanium OC76 and an OC140 germanium transistor in black painted glass envelope. > > http://panteltje.com/pub/old_germanium_transistors_IMG_6554.JPG > > I have some CK722's! > > https://www.dropbox.com/s/lq8w4jsh0sn5fc0/Ck722-0A.JPG?raw=1 > > and some high-voltage opamps > > https://www.dropbox.com/s/st1upzhqkil8bp6/Philbricks.jpg?raw=1
I've got a tray full of HV opamps from Apex (PA341). Too noisy for me but if anyone wants some drop me a line. Maybe I should try ebay? George H.
> > > > > > Here's my oscillator: > > https://www.dropbox.com/s/19imyfg1ubh2z3c/P5_CCRO.jpg?raw=1 > > The CCROs have TCs, Qs, and initial accuracies over 10x better than an > LC. The higher-frequency parts have Qs in the thousands. > > > > > -- > > John Larkin Highland Technology, Inc > picosecond timing precision measurement > > jlarkin att highlandtechnology dott com > http://www.highlandtechnology.com