simple dual transistor amplifier - Page 1
Forum section: electronics / projects

author: robse
author_rank: Contributor
timestamp: April 07, 2018, 10:07:46 pm
content: Hello everybody,I have this project of a dual transistor amplifier. I made it on pcb and tested with a waveform generator + 4ch oscilloscope.It has a voltage gain of about 10 under 500kHz, and this is what I want.But I see that the output waveform doesn't go below about 2.65V (there it clips). Can I improve that (possibly without adding components since I already made the boards) ?On the image below you see that I get the "yellow signal" (that is in phase with the input) and the "green signal" (that is 180 degree out of phase). This is ok, and ideally these two phases should be simmetrical. R4 is a trimmer that should align the levels of these two signals (it does the job but not enough)Thank you,Robse

author: BrianHG
author_rank: Super Contributor
timestamp: April 07, 2018, 10:23:56 pm
content: Shrink R3. The smaller R3, the larger your available output swing area. Note that your signal gain will increase wildly. R3 can go as low as 0 ohm for full 0-5v output swing, but, your sine wave will begin to distort. You will also loose your symmetrical + and - output balance.The other choice is to give the GND side of R3 a negative 5v supply and you will keep a balanced + & - output signal, one going from 0v to 5v and the other going from -5v to 0v.Adjust R7 for your signal gain.

author: robse
author_rank: Contributor
timestamp: April 07, 2018, 11:40:28 pm
content: Putting a -5V in the circuit would be too much complicated for this part of the project, so that's not an option.with 100ohm R3 the yellow output stays at around 2.0V and the green phase at about 0.6V.Multisim seems to behave not good in this situation, if I stop and re-run the simulation I get zero amplification if R4 is not set at 50%The guy that gave me the project said that R4 was needed for simmetrical output (phase+/phase-), but it looks like it is not enoughmmm

author: BrianHG
author_rank: Super Contributor
timestamp: April 08, 2018, 12:01:59 am
content: If you need both phases to be the same output voltage level p-p, and you need a larger voltage level p-p, you have no choice but to increase the supply voltage to get a larger output swing.With a matched 470 ohm above and below the transistor (ie on emitter and collector), when TR2 is completely off, those 2 outputs can only go to their voltage rails, 1 at GND, the other at VCC, and when the transistor is fully on, closes, making a short between the 2, the voltage at the emitter and collector will both be near the center voltage of 2.5v. Actually, it will be a little higher since feeding the base current to turn on the transistor is pulling the whole circuit up by a little bit. Shrinking R5 may pull the transistor further on, but, this will also pull the center of that whole circuit up a little further. Only changing TR2 with a MOSFET will give you that full 0v-2.5v, and 5v-2.5v outputs, but, the signal wont be linear as it gets close to the 2.5v center.Increasing the total supply voltage may be the only way to give you more output swing area.Now, I'm hoping I didn't get your problem backwards.

author: StillTrying
author_rank: Super Contributor
timestamp: April 08, 2018, 01:37:49 pm
content: Setting the yellow output to 3.8V, and the green output to 1.2V will be the very best that you can out of that TR2 phase splitter, giving close to 2V pk-pk outputs.