# How to select transistor for oscillator ?

Started by September 11, 2018
```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 ?

```
```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.
where Zo is the resonant impedance, which will be Q times the equivalent
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
> 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 ?
>

```
```On Tuesday, 11 September 2018 04:38:52 UTC+1, daku...@gmail.com  wrote:
> 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 ?
>

in most cases it just needs enough gain at the chosen frequency. How much gain do you need? Other parameters sometimes matter of course.

NT
```
```On Mon, 10 Sep 2018 20:38:48 -0700 (PDT), dakupoto@gmail.com wrote:

>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 ?
>

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.

--

John Larkin         Highland Technology, Inc

lunatic fringe electronics

```
```On 09/11/2018 11:36 AM, John Larkin wrote:
> On Mon, 10 Sep 2018 20:38:48 -0700 (PDT), dakupoto@gmail.com wrote:
>
>> 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 ?
>>
>
>
> 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.
>
>

They're just too "unstable" HEH
```
```On 09/10/2018 11:38 PM, dakupoto@gmail.com wrote:
> 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 ?
>
>

5 MHz? is a 2N3904 not good enough for some reason?
```
```On 09/11/2018 09:26 AM, tabbypurr@gmail.com wrote:
> On Tuesday, 11 September 2018 04:38:52 UTC+1, daku...@gmail.com  wrote:
>> 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 ?
>>
>
> in most cases it just needs enough gain at the chosen frequency. How much gain do you need? Other parameters sometimes matter of course.
>
>
> NT
>

This kind of detailed thinking is what I might go thru to build a 500
MHz oscillator, not a 5 MHz oscillator. For 5 MHz just find Colpitts
schematic online it will probably use a 2N3904. select components in
tank for frequency and simulate in LTSpice. Tweak values if needed.
Build circuit on small slab of copper clad it will work fine 95% chance.
Can be all done in the time one takes to discuss it here...
```
```On 09/11/2018 11:36 AM, John Larkin wrote:
> On Mon, 10 Sep 2018 20:38:48 -0700 (PDT), dakupoto@gmail.com wrote:
>
>> 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 ?
>>
>
>
> 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.

Amplifiers like to oscillate very much in the proper configuration and
if they're in that configuration the tolerances can be pretty broad they
don't need math to cajole them.
```
```On Tue, 11 Sep 2018 13:16:12 -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:
>>
>>> 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 ?
>>>
>>
>>
>> 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.
>>
>>
>
>They're just too "unstable" HEH

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.

--

John Larkin         Highland Technology, Inc
picosecond timing   precision measurement

jlarkin att highlandtechnology dott com
http://www.highlandtechnology.com

```
```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

```