featherpillow
Well-known member
I did a preliminary search for this, but couldn't find anything. If this question has already been answered in another thread, just point me to it...
Typically, we operate a differential from a bipolar supply, but the circuit shown uses a single-sided supply (I guess it's similar to PRR's different differential in that respect).
When putting this circuit together, Horowitz and Hill tell me that R3 should be chosen to give about 100uA to the collector of Q2, and that no collector resistor is needed for Q1 because no output is taken from there. R1 provides a reference voltage for the transistor bases, correct? And R2 sets input impedance...
They tell me that gain is calculated as R3/(2 x R6+R4).
Third, they tell me that CMRR is roughly R6/(R4+R5).
Fourth, R6 is chosen to give emitter current of about 200uA, but maybe that doesn't matter with a single sided supply...?
Given these, I'm trying to figure out a way to put this circuit together with relatively high CMRR and unity gain. If I omit R4 and R5 entirely, differential input impedance drops, and the differential gain goes up (if I understand this part of the analysis correctly--I'm not sure that I do). If I increase their size, CMRR goes down. Do you see my conundrum?
If I substitute a current source for R6, I can substantially improve CMRR, of course. But how can I do that with a single sided supply?
Typically, we operate a differential from a bipolar supply, but the circuit shown uses a single-sided supply (I guess it's similar to PRR's different differential in that respect).
When putting this circuit together, Horowitz and Hill tell me that R3 should be chosen to give about 100uA to the collector of Q2, and that no collector resistor is needed for Q1 because no output is taken from there. R1 provides a reference voltage for the transistor bases, correct? And R2 sets input impedance...
They tell me that gain is calculated as R3/(2 x R6+R4).
Third, they tell me that CMRR is roughly R6/(R4+R5).
Fourth, R6 is chosen to give emitter current of about 200uA, but maybe that doesn't matter with a single sided supply...?
Given these, I'm trying to figure out a way to put this circuit together with relatively high CMRR and unity gain. If I omit R4 and R5 entirely, differential input impedance drops, and the differential gain goes up (if I understand this part of the analysis correctly--I'm not sure that I do). If I increase their size, CMRR goes down. Do you see my conundrum?
If I substitute a current source for R6, I can substantially improve CMRR, of course. But how can I do that with a single sided supply?