Silly PSR question in diff pair stage

[geozog86]

Hello!

After simulate this simple amp (all properly biased), I simulated the graph on the right hand side: Looks like for the low freqs, it's got a PSR of 0dB (ac disturbance in supply, measuring ac at the output).

Isn't that weird? Doesn't 0dBs mean that transistor 3 acts as a resistor? Can someone explain to me the theory behind that? Could be something really basic that I neglect/haven't understood. There was no load in the simdeck (it was just the circuit drawn, nothing connects to output)

I AM CONFUSED

Thx

 

[D.A.(Tony)Stewart]

Current source on bottom is essential to PSRR

 

[geozog86]

Ok, but how does that affect the 0dBs PSRR at low frequencies? I thought it'd play an important role at higher frequencies, no?

 

[D.A.(Tony)Stewart]

Its a current source so high impedance decoupling from V+ for all f including DC

 

[geozog86]

Thx for the reply, I completely neglected the effect of the current source, but I still do NOT understand why transistor 3 acts as an open switch and just lets all ac disturbance on positive supply pass on the output.