How do I find the common-mode rejection ratio, CMRR for this circuit? I know I have to find differential gain and common-mode gain as 'differential mode gain/commonmode gain' but with current mirrors in, I'm having trouble finding it.. Could not find though I saw other questions posted here Thank you
CMRR = 20 * log (|Gd| / |Gcm|)
Gd = differential mode gain
Gcm = common mode gain
Let's calculate Gcm:
Add two identical voltage sources Vs1(t) to the circuit: the first on VINP and second on VINN.
We have a common mode signal applied to the circuit.
Superposition now:
Turn off (short VINN to GND) the second and calculate:
G1p(t) = VTIA'(t) / Vs1(t) or G1p(s) = VTIA'(s) / Vs1(s)
Now turn off (short VINP to GND) the first and calculate:
G1n(t) = VTIA''(t) / Vs1(t) or G1n(s) = VTIA''(s) / Vs1(s)
Now calculate Gcm:
Gcm(s) = G1n(s) + G1p(s)
or
Gcm(t) = Gn(t) + Gp(t)
Let's calculate Gd:
Add two identical voltage sources Vs2(t) to the circuit: the first on VINP and second on VINN.
This time though flip vertically Vs2(t) on VIN in order to have a differential signal applied to the circuit.
Superposition now:
Turn off (short VINN to GND) the second and calculate:
G3p(t) = VTIA'(t) / Vs2(t) or G2p(s) = VTIA'(s) / Vs2(s)
Turn off (short VINP to GND) the first and calculate:
Gn2(t) = VTIA''(t) / Vs2(t) or Gn2(s) = VTIA''(s) / Vs2(s)
Now calculate Gcm:
Gcm(s) = G2n(s) + G2p(s)
or
Gcm(t) = G2n(t) + G2p(t)
Finally calculate:
CMRR = 20 * log (|Gd| / |Gcm|)
