Sorry it this is kind of a basic question. I'm trying to get my head around common circuit designs for three stage amplifiers.
I have seen several schematics where the current source that feeds the LTP has a capacitor from B+ to a voltage divider between ground and the base/collector. An example of this is C3 and R7/R9 in the attached example.
As far as I understand, the purpose of this is to stabilize the bias point of the transistors making up the CCS, but if someone could elaborate a bit more around this circuit block, it would be really helpful. What is the benefit of having the capacitor connected to a voltage divider here? To improve PSRR?
Thanks!
I have seen several schematics where the current source that feeds the LTP has a capacitor from B+ to a voltage divider between ground and the base/collector. An example of this is C3 and R7/R9 in the attached example.
As far as I understand, the purpose of this is to stabilize the bias point of the transistors making up the CCS, but if someone could elaborate a bit more around this circuit block, it would be really helpful. What is the benefit of having the capacitor connected to a voltage divider here? To improve PSRR?
Thanks!
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Hi there,
It does help improve the power supply rejection ratio: If you consider what happens when some ripple voltage happens on the B+ rail: that ripple, because it is a.c. passes through C3 to the mid point of the two 10K resistors and up to the collector of Q1. In effect, it opposes any voltage change across Q1 (part of the constant current source) because you have an in-phase voltage 'pinching' across it. Its a sort of bootstrapping method.
Doug Self shows it in his amplifier designs as a useful technique.
Kind regards
Mike
It does help improve the power supply rejection ratio: If you consider what happens when some ripple voltage happens on the B+ rail: that ripple, because it is a.c. passes through C3 to the mid point of the two 10K resistors and up to the collector of Q1. In effect, it opposes any voltage change across Q1 (part of the constant current source) because you have an in-phase voltage 'pinching' across it. Its a sort of bootstrapping method.
Doug Self shows it in his amplifier designs as a useful technique.
Kind regards
Mike
Thank you! That clarifies it. I assumed that the purpose of the capacitor was mainly to pass AC from the B+ rail to Q1, but now I got some more pieces to the puzzle! So then the voltage divider is to give the ripple the correct DC-potential for Q1?