Is it enough to connect replace the PNP transistors with NPN and put a positive VCC where a negative exists now?
Something like this?What I would propose is you try this out: use a single transistor amplifier stage, input fed to the base and output from the collector, you can use NPN or PNP as long as you provide the appropriate supply polarity. It needs a reasonably high emitter resistor to work, I would suggest 4.7K as a good starting value and it must not be bypassed with a capacitor as you need signal on the emitter for this to work. Then wire a twin-tee network between the base and emitter using two 10K series resistors with 44nF to ground and two 22nF capacitors with 5K to ground. Those values will give a peak at about 750Hz which is nice for CW. Note the values need to be exact but you can make 5K from two 10Ks in parallel and 44nF from two 22nF in parallel.
See what happens and let me know. If it shows promise, there is a simple modification that allows true bandwidth adjustment.
Brian.
hmm.... I would add a collector load resistor or the output will be Vvveeerrrryyyy quiet!
Yes, miss R2 out and calculate R1 according to the transistor Hfe.
The problem with such a simple implementation is that for best results, the input impedance should be very low and the output load very high impedance, both of which this isn't able to satisfy. Try it though and see how it performs. Remember the R/2R and C/2C rule and make the values by parallel resistors/capacitors rather than using nearest values.
Brian.
What might work would be a Wein bridge oscillator circuit, held just below the threshold of oscillation.
Just as an experiment while you have it built, try injecting the signal at the emitter instead of the base. I have no idea if it will make any difference but it makes it a common base amplifier which might exhibit slight gain.
Brian.
Similar idea, but I would do it differently.
View attachment 135135
Tweak the pot until it oscillates at 1Khz, then back it off just below the threshold of oscillation.
It should have plenty of gain there at 1Khz.
To be really effective, I think it would need it to be within a hair of oscillating.
That is about the best you can probably do with only a single stage.
The Wein bridge has positive feedback, and potentially a much taller and narrower peak. But its tricky and potentially unstable.
Only other consideration might be that the Wein is easier to tune requiring only two equal value resistors, which is dead easy. If that is a required feature.
The bridged tee, requires three tunable resistors, which becomes a bit more difficult.
That is what I feared. In fact the feedback path from the emitter has no voltage gain at all. When I've used it in the past it has been in the feedback of op-amp circuits where the gain is high and the impedances better controlled. It might be possible to add gain by using a second transistor stage but as the function of the twin-tee is a notch you have to be careful to not to invert the signal while amplifying the voltage, that may not be so easy.
Just as an experiment while you have it built, try injecting the signal at the emitter instead of the base. I have no idea if it will make any difference but it makes it a common base amplifier which might exhibit slight gain.
Brian.
The purpose of the Q1 stage is to present a constant impedance at the summing point (base of Q3) but depending on what you feed it from, you may be able to leave that out too.
Brian.
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