how to calculate the input impedance of this circuit?

[katrin]

calculating input impedance

Now I want to calculate the input impedance of a circuit, the input signal is a current signal, the circuit's input is composed of a complementary PMOS and NMOS common gate amplifier(M1 and M3)

M2, R2 and M4, R4 provide constant voltage bias for M1 and M3

If I want to do detailed analysis of input impedance, Vin/Iin, in other words, to derive the relation between Vin/Iin and the circuits parameters such like Gm, rds....

I think mainly I should consider about M1 and M3, but I am not sure if I can neglect the other components in the calculation.

I hope someone who is familiar with circuits analysis can help me with that, thanks.

 

[Blackuni]

calculate input impedance

Hi,


If u can use spice then do AC analysis by connecting a AC current source at your inputs. Do frequency sweep. Normally your ac current source will be 1 A. so voltage at your input port is nothing but the effective small signal impedance seen at your input port.

This value will be much accurate than any other.

Thanks,

 

[diemilio]

input impedance calculation

For hand calculations you should consider M1, M3 and also R1 and R3; you will be able to neglect certain things depending on their values. The complete expression will be given by:

vin/iin = ((ro3+R3)/(1+gm3*ro3))||((ro1+R1)/(1+gm1*ro1))

if gm*ro<<1

vin/iin = (1/gm3+R3/(gm3*ro3))||(1/gm1+R1/(gm1*ro1))

Depending on the ration R/ro you will be able to neglect (or not) other terms.

Hope this helps,

diemilio

 

[ytliang]

how to calculate impedance

you didnt specify where the source of the transistor is. I am assuming the input port is connected to M1 and M3's source. In that case, your input impedance should be very small ( ~ 1/gm)

 

[diemilio]

how to calculate input resistance

you didnt specify where the source of the transistor is. I am assuming the input port is connected to M1 and M3's source. In that case, your input impedance should be very small ( ~ 1/gm)

As as mention before, this is true only if you can neglect the R/(gm*ro) term. If you can do this, then your impedance is NOT 1/gm, but (1/gm1)||(1/gm3), which (if your transistors are ratioed 2:1) will be 1/(2*gm)

diemilio

 

[visarg.shah]

can we find the input impedance of a cro from its minimum ferquncy and volteg value