Common collector Colpitts oscillator frequency formula

For starters, have a look at .

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-TV

Schematics would help us understand what the problem is.

Apart from the effects of parasitic capacitances, which I did not attempt to analyze, the same. The tank is still a single L in parallel with a series pair of C's, even if that may not be immediately obvious from the schematic diagram.

Removing all secondary considerations, I think of a Colpitts as basically this:

+---L---+----+ | | | | |/ | +-----| C | |\ e | | | | +---C---+----+

You then choose a ground point and re-arrange and add components to get the biasing right without upsetting the AC equivalent circuit.

If you exchange L's for C's and the reverse, it would be a Hartley oscillator. If you replace the L by a series L-C, it would be a Clapp. If you replace the L by a quartz crystal, it would be a Pierce. There are more.

Jeroen Belleman

Not a gure, but looked at exactly this problem. In first approximation regardless of oscilator type (Colpitts, Hartey, common emiter, common base, common collector) you have frequency of resonant tank. If you need more accurate results you need to look at parasitics and imperfections of elements. Most of the time it is enough to pretend rest of circult is perfect, but L and C have different value. For accurate results you would need real guru. But beware: apparently nobody can accurately compute amplitude of oscilation and it would be extremally weird if freqency were completely independent from amplitude. To be clear: dependence on amplitude is certainly smaller than other effects like change of transistor parameters with temperature, so for most practical purposes is negligible. But again, for most practical purposes frequency of resonant tank is good enough.

To state the obvous: in resonat tank the two capacitors form series connection, so you use formula for resulting capacitance. Also, in common collector Colpitts connecting emiter between capacitors give you effect of transformer: higher voltage on base. Common collector has voltage gain less than 1, so this "transformer effect" is essential to get oscilations.

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                              Waldek Hebisch

You set the amplitude by increasing the current to the oscillator. For a common collector Colpitts, the amplitude will change very little from one device to another. The common collector Colpitts also has the advantage that the feedback capacitors tend to swamp out any changes in device parameters.

There are a number of techniques to reduce the 1/f noise. These will also tend to stabilize the amplitude.

Changing the transistor type can have a large effect on the amplitude, especially if you are running at high frequencies.

For a Pierce oscillator, you change the signal into the tank by changing the feedback resistor.

For examples, see

Oscillator.zip

"It is dark. You are likely to be eaten by a gure."

:^)

Tim

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Seven Transistor Labs, LLC 
Electrical Engineering Consultation and Design 
Website: https://www.seventransistorlabs.com/

Yes, I fully agree with you say. My problem is that I am examining an old common collector Colpitts oscillator design with a target oscillation frequency of 75 MHz. The engineer who designed it has moved to a different company some tears ago. The value of each of the two capacitors in the series pair is 39pF, and the inductor is 0.2uH. So the equivalent capacitance is 19.5pF, and usig expression: fosc = 1/(2.0*PI*sqrt(L*Ceqv)) the numerical value of the oscillation frequency is: fosc = (10^9)/(2*PI*sqrt(3.9)) = 80.0MHz. The SPICE netlist, when simulated gives oscillation frequency of 77 MHz. So far, so good. The design is a bit unusual as the inductor is connected directly to the BJT base biasing voltage divider, with a

1nF capacitor at the same node to ground, to suck out AC. The problem is that if I try to use a different fosc, e.g., 150 MHz, and compute a different set of component values(for this new oscillation frequency) the oscillator latches up. To design the common mode amplifier, I use the standard design rules Ve is approx. 0.5Vcc, Vb = Ve + 0.65 and maximum biasing current is 10x(Ic/beta(min)), where it is assumed that Ic is approx. equal to Ie, which in turn is set by the designer. I am using 2N5179, for which beta(min) = 25, and Ic(max) = 50mA. The amplifier stage simulates perfectly fine.

So, what could be causing the latchup ? Any hints/suggestions ?

Watch out yourself. Halloween is approaching -- nights are colder and darker and 'gures' have the irritating habit of popping up at the most unexpected places.

Sounds like you miss him...

I have trouble seeing how such a simple circuit can latch up. What are the node voltages when that happens?

Did you bypass the collector to GND?

Jeroen Belleman

If this is a simulation, you often need to set up some unbalanced initial condition to make the oscillation start up.

In real circuit, there is enough noise which helps the start-up.

--

-TV

Tauno Voipio wrote:

This is simply not true. SPICE has numerical residuals at low levels that provide the noise needed to start the oscillation. If the loop gain is greater than one, and the phase shift is a multiple of 360 degrees, the circuit will oscillate. The startup and settling time is determined by the feedback gain and the circuit Q.

Oscillator design is trivial if you follow the simple instructions in my article at

It shows the original 75MHz oscillator is not optimum. The inductor has a reactance of 2*pi*0.2e-6*75e6 = 94.24 Ohms.

This is twice the value that is needed. The penalty is lower Q due to skin effect and greater sensitivity to stay capacitance in the pc layout, such as trace capacity and component connections. So XL = 50 Ohms is a better choice.

The attempt to make a 150MHz oscillator without following the simple instructions in my article is probably doomed to failure.

I include a simple Colpitts that runs at 153.7MHz since I didn't bother including the bias filter capacitor C3, of 200pF in the capacitor calculations. In the actual circuit, stray capacitances will lower the oscillation frequency, so trimming will be required.

The oscillator does not require any pertubation to start oscillating. It starts by itself. If you do have LTspice, you can download it at

The XVII version will not run on Win XP.

Here is the ASC file:

Version 4 SHEET 1 880 708 WIRE -800 -96 -896 -96 WIRE -448 -96 -800 -96 WIRE -224 -96 -448 -96 WIRE -896 -64 -896 -96 WIRE -800 -64 -800 -96 WIRE -224 32 -224 -96 WIRE -896 48 -896 16 WIRE -800 80 -800 16 WIRE -752 80 -800 80 WIRE -736 80 -752 80 WIRE -672 80 -736 80 WIRE -560 80 -592 80 WIRE -528 80 -560 80 WIRE -400 80 -448 80 WIRE -336 80 -400 80 WIRE -288 80 -336 80 WIRE -800 112 -800 80 WIRE -736 112 -736 80 WIRE -400 160 -400 80 WIRE -800 208 -800 192 WIRE -736 208 -736 176 WIRE -400 240 -400 224 WIRE -224 240 -224 128 WIRE -224 240 -400 240 WIRE -128 240 -224 240 WIRE -80 240 -128 240 WIRE -224 256 -224 240 WIRE -400 272 -400 240 WIRE -400 352 -400 336 WIRE -224 352 -224 336 FLAG -896 48 0 FLAG -128 240 vout FLAG -800 208 0 FLAG -736 208 0 FLAG -400 352 0 FLAG -224 352 0 FLAG -448 -96 VCC FLAG -752 80 R1C3 FLAG -560 80 R4L1 FLAG -336 80 Q1B SYMBOL voltage -896 -80 R0 WINDOW 123 0 0 Left 2 WINDOW 39 0 0 Left 2 SYMATTR InstName V1 SYMATTR Value 5 SYMBOL res -816 -80 R0 SYMATTR InstName R1 SYMATTR Value 47k SYMBOL npn -288 32 R0 SYMATTR InstName Q1 SYMATTR Value 2N5179 SYMBOL cap -416 160 R0 SYMATTR InstName C1 SYMATTR Value 42.4pf SYMBOL res -240 240 R0 SYMATTR InstName R2 SYMATTR Value 2k SYMBOL cap -416 272 R0 SYMATTR InstName C2 SYMATTR Value 42.4pf SYMBOL ind -544 96 R270 WINDOW 0 32 56 VTop 2 WINDOW 3 5 56 VBottom 2 SYMATTR InstName L1 SYMATTR Value 53nH SYMATTR SpiceLine Rser=0 SYMBOL cap -752 112 R0 SYMATTR InstName C3 SYMATTR Value 200p SYMBOL res -816 96 R0 SYMATTR InstName R3 SYMATTR Value 47k SYMBOL res -576 64 R90 WINDOW 0 0 56 VBottom 2 WINDOW 3 32 56 VTop 2 SYMATTR InstName R4 SYMATTR Value 2.5 TEXT -664 -136 Left 2 !.tran 0 500n 0 5p TEXT -664 -160 Left 2 ;'150MHz Colpitts LC Osc

One of the keys is to make sure that it's emitter-base cutoff that does the amplitude limiting, rather than collector saturation.

Cheers

Phil Hobbs

--
Dr Philip C D Hobbs 
Principal Consultant 
ElectroOptical Innovations LLC / Hobbs ElectroOptics 
Optics, Electro-optics, Photonics, Analog Electronics 
Briarcliff Manor NY 10510 

http://electrooptical.net 
http://hobbs-eo.com

He's "in a maze of twisty little passages, all alike." ;)

Cheers

Phil Hobbs

--
Dr Philip C D Hobbs 
Principal Consultant 
ElectroOptical Innovations LLC / Hobbs ElectroOptics 
Optics, Electro-optics, Photonics, Analog Electronics 
Briarcliff Manor NY 10510 

http://electrooptical.net 
http://hobbs-eo.com

Oh, the horrors of an uppercase D!

Be gentle, but carry a +5 club!

Gerhard

I detect another misspent youth. ;)

Watch out for the pirate!

Cheers

Phil Hobbs

--
Dr Philip C D Hobbs 
Principal Consultant 
ElectroOptical Innovations LLC / Hobbs ElectroOptics 
Optics, Electro-optics, Photonics, Analog Electronics 
Briarcliff Manor NY 10510 

http://electrooptical.net 
http://hobbs-eo.com

You adjust the amplitude of oscillation by changing the emitter resistor.

I don't know how the emitter-base cutoff can affect the amplitude. The transistor is turned off when the emitter-base junction is reverse biased.

I mention to monitor the emitter-base reverse bias and the VCC-base voltage in the notes.

In good designs, absolutely. In bad ones, the amplitude is limited by collector saturation.

Cheers

Phil Hobbs

--
Dr Philip C D Hobbs 
Principal Consultant 
ElectroOptical Innovations LLC / Hobbs ElectroOptics 
Optics, Electro-optics, Photonics, Analog Electronics 
Briarcliff Manor NY 10510 

http://electrooptical.net 
http://hobbs-eo.com

If you mean the base voltage reaches VCC on the positive peaks, I specifically mention to avoid this in the notes.

To avoid it, reduce the amplitude of oscillation by increasing the emitter resistor.

This reduces the energy fed into the tank and results in lower amplitude. However, the transistor is still off most of the time, so I don't know what you mean when you say "it's emitter-base cutoff that does the amplitude limiting."

I have already extracted and read through the README.txt file that is included in your oscillator.zip. As I use HSpice and Ngspice, the LTSpice files would not be very useful, but instructions in the README file would definitely be useful.Thank you.