- Nov 24, 2005
- Reaction score
- Amsterdam, The Netherlands
(this is just a concept; phantom, supply decoupling, RFI filtering and the like still need to be added)
The differential op-amp is a TI OPA1632 or similar. This part will keep the common-mode voltage at its output at Vocm, which is grounded in this design. Here also lies the main difference with the familiar Cohen/Green input stage: where the Green will reproduce any common-mode voltage at its input at the output (or Acm = 1 for all settings of the gain network), this configuration should have an Acm of 0.
The trouble with that is that the emitter current of the input transistors will be modulated by any common mode signal present at the input. For perfectly matched transistors this should have no negative impact, but I'm not entirely sure what will happen when the transistors are mismatched.
[need to sim this, but can't find any good diff amp models]