# Finding Vds in a CMOS without info. on channel modulation?

"What would (ideally) be the DC voltage at the output in the following circuit?" That's how the question was formulated. Can it be found without having any information on $V_A$ or on $\lambda$? The only way $v_o$ could be evaluated would be by equating the currents through Q6 and Q7, but that would only yield $v_o$ provided that I have some information on either $V_A$ or on $\lambda$. Could anyone help me settle it?

• How about posting a plot of the I_V charactieristic, for the gate-length(s) used in your amplifier? – analogsystemsrf May 27 '17 at 14:13
• I just posted all the information given in the question. I have no other info. – peripatein May 27 '17 at 14:14
• Oh, and in a previous section I was asked to perform a DC design that will result in each of $Q_{1-4}$ and $Q_{6,7}$ conducting a drain current of $200\mu A$ and all transistors operating at $V_{ov}=0.2V$. I managed that. – peripatein May 27 '17 at 14:17
• Are Va and Vb given, Hint its easy for the two cases of Va>Vb or Va < Vb – sstobbe May 27 '17 at 15:19
• current mirror and differential V I/O determine inverse transfer function with k n/p ratios for RdsOn but matched Vth – Tony Stewart Sunnyskyguy EE75 May 27 '17 at 15:22

The OP provided a sample solution to the problem as,

Turning to spice, we can determine the complete hypothetical solution as,

Spice estimates the output to be, $$Vo \simeq -50 \textrm{ mV}$$

The practical aside is if M4 is 1% larger than expected, the input offset voltage causes the output to rail.

• how do I find it in my problem given that I was asked for the ideal output voltage without specifying lambda and that the current is 200uA through Q1-4 and through Q6-7? I can give you the W/L's I found, should that help, as they are different than the ones here – peripatein May 27 '17 at 20:20
• You have it in the solution you provided, just assume $\lambda_N = \lambda_P$, and it divides out. – sstobbe May 27 '17 at 20:26
• So say I found $(W/L)_{1,2,7,8}=25, (W/L)_5=50,(W/L)_{3,4,6}=100$, how would that affect the output voltage? Mind you that I also have no idea what the value of lambda is, unlike the question above – peripatein May 27 '17 at 20:37
• It won't divide out as the W/L's are different.. Moreover, why does the question specifically ask for the "ideal" solution? What does "ideal" mean here? rds equals infinity? – peripatein May 27 '17 at 20:51
• Spice solution for full opamp -92 mV, just the matched output stage 0 V. – sstobbe May 27 '17 at 20:56