transistorized differential amplifiers

Thread Starter

stepank

Joined Apr 19, 2018
8
hello aac-
i am a student, taking an intro physics class in electronics - but the teacher always tells me to "go ask an engineer"..so i came here.

i have a working understanding of how differential amplifiers work in operational amplifier IC arrangements - but they seem to act very differently in a transistorized circuit. i am attempting to translate an op amp circuit into the transistor domain and failing at it.

essentially, i would like to take a bipolar sine (+/- 12v) and add enough DC offset so that only the peak of the sine would reach the output. then, i would like to use the other end of a differential amplifier and feed it the offset (without the sine), so that the peak would center itself on ground. Presumably, the circuit would look something like this: http://tinyurl.com/ycktghlg

but this circuit falls quite short. can anyone here help me understand what to do?
 

Thread Starter

stepank

Joined Apr 19, 2018
8
Sorry about that: this is the simulation I meant to post:
http://tinyurl.com/yblk36oe

This seems to work, actually. But I would be open to suggestions and help understanding if there is more I can do to insure the stable operation of this circuit, centered around ground. I notice that it doesn't seem to stay fixed around the point, depending on the bias and frequency...and that I can't quite trim it to be perfect... Are there tried&true methods of centering a transistor output?
 

Hymie

Joined Mar 30, 2018
997
Sorry about that: this is the simulation I meant to post:
http://tinyurl.com/yblk36oe

This seems to work, actually. But I would be open to suggestions and help understanding if there is more I can do to insure the stable operation of this circuit, centered around ground. I notice that it doesn't seem to stay fixed around the point, depending on the bias and frequency...and that I can't quite trim it to be perfect... Are there tried&true methods of centering a transistor output?
I suggest you take a look at a data sheet for an op-amp available online, many include the circuit schematic within the IC, which should assist with the typical differential input transistor arrangement.
 

Thread Starter

stepank

Joined Apr 19, 2018
8
hi -
thanks for the suggestions - but i just want to clarify, i'd like to operate this without an op-amp - or even necessarily building a discrete transistor op amp... my feeling is the general differential amplifier circuit, shared above, works pretty well - but there's a question about biasing the output of a transistor so that it falls exactly where i want... instead of using hyper-precise resistor networks, is there a general circuit for "centering' (having the output rest on ground, or even having the output using ground as a center-crossing) the output of a transistor?
 
Top