differential amplifier

[Andrew Xiang]

If I make a differential amplifier with 2 BJTs, But I only take output
from one collector? I know the diff gain is only half, but do I still
get the same benefit of CMRR?

Want to do a balanced mic preamp if it is beneficial. My wire is only
10" , I wonder if it makes much difference?

-Andrew

 

[Jim Thompson]

Hi Kevin:

I agree with your answer but don't understand your approximation. The CMRR
is typically in the range of 100 dB which would indicate an Early voltage of
2.5 kV.

(1) CMRR is usually limited by the performance of the stage where you
translate from differential to single-ended.

(2) The input current mirror behaves better than what would be
indicated by VAF... it is, after all, usually degenerated with emitter
resistors, which raises the output impedance.

...Jim Thompson

 

[Charles Schuler]

(1) CMRR is usually limited by the performance of the stage where you
translate from differential to single-ended.

(2) The input current mirror behaves better than what would be
indicated by VAF... it is, after all, usually degenerated with emitter
resistors, which raises the output impedance.

Hi Jim:

The approximation that I am familiar with for the impedance of the current
source used with some differential amplifers is:

rcs = rc(1+Re/re)

where rc is a function of the Early voltage and Re is the emitter resistor
in the current source.

 

[Jim Thompson]

Hi Jim:

The approximation that I am familiar with for the impedance of the current
source used with some differential amplifers is:

rcs = rc(1+Re/re)

where rc is a function of the Early voltage and Re is the emitter resistor
in the current source.

Your rule of thumb seems about right... but I don't fret the
approximations, I live in the monolithic world, I *simulate* over all
process corners and operating temperatures ;-)

When I need *really* stiff current mirrors I use cascodes.

...Jim Thompson

 

[Kevin Aylward]

Hi Jim:

The approximation that I am familiar with for the impedance of the
current source used with some differential amplifers is:

rcs = rc(1+Re/re)

where rc is a function of the Early voltage and Re is the emitter
resistor in the current source.

This is correct up to the limit set by hfe. The maximum will be ro.hfe,
for example, if Re is the collector of another transistor.

Kevin Aylward
[email protected]
http://www.anasoft.co.uk
SuperSpice, a very affordable Mixed-Mode
Windows Simulator with Schematic Capture,
Waveform Display, FFT's and Filter Design.