Hello, I am attempting to design a resistivelyloaded differential pair amplifier.
The specs are listed below:
1. Power consumption < 50 mW
2. Differential gain = 20 dB
3. Input common mode range = 5 to 9 V
4. VDD = 12 V
As you can see in the attached circuit drawing, the only parameters that I have to design are the current source Ibias and the offset voltage of the inputs.
My attempt at the solution is shown below:
Gain = 20 dB ==> 10 V/V
Get 5 V/V from each side of diff pair
A = gm(R1ro1) = 5 V/V
A = gm(10k54.47k) = 5 V/V
gm = 0.592 mA/v
I = (gm)^2 / (2*kn) = (0.592mA/V)^2 / (2*0.5934) = 0.295 mA = 0.3 mA
Ibias = 2*I = 0.6 mA
Now I attempt to determine the offset voltage for the inputs.
1. To be in saturation, Vds > Vgs  Vt
2. Therefore Vd > Vg  Vt
3. Then Vg < Vd + Vt
4. Vd = VDD  I*R1 = 12V  (.3mA)(10k) = 12V  3V = 9V
Then from (3), Vg < Vd + Vt
Vg < 9 V + .7 V = 9.7 V
So Vg, the gate voltage of U4, must be less than 9.7 V.
I choose 6.7 V for the gate voltage to get well below the edge of saturation.
This is what I have so far. I then attempt to do a DC sweep analysis to find the transfer characteristic of (VO2VO1) vs (VINP  VINN). Once I have the transfer characteristic, I will find the maximum linear region and compute the DC gain.
My problem is that I am not sure my procedure for doing the DC sweep of a differential amplifier is correct. I have attempted to use a parameter Vbias (as can be seen in the circuit) to sweep both input offset voltages simultaneously. The waveform I get though after placing a probe at one of the amplifier's outputs shows that the input offset bias should be around 3.5 V (in the linear region), which was not what I calculated (~6.7 V).
Any help regarding whether my calculations are wrong or whether I am doing the DC sweep incorrectly to find the transfer characteristic would be greatly appreciated.
The specs are listed below:
1. Power consumption < 50 mW
2. Differential gain = 20 dB
3. Input common mode range = 5 to 9 V
4. VDD = 12 V
As you can see in the attached circuit drawing, the only parameters that I have to design are the current source Ibias and the offset voltage of the inputs.
My attempt at the solution is shown below:
Gain = 20 dB ==> 10 V/V
Get 5 V/V from each side of diff pair
A = gm(R1ro1) = 5 V/V
A = gm(10k54.47k) = 5 V/V
gm = 0.592 mA/v
I = (gm)^2 / (2*kn) = (0.592mA/V)^2 / (2*0.5934) = 0.295 mA = 0.3 mA
Ibias = 2*I = 0.6 mA
Now I attempt to determine the offset voltage for the inputs.
1. To be in saturation, Vds > Vgs  Vt
2. Therefore Vd > Vg  Vt
3. Then Vg < Vd + Vt
4. Vd = VDD  I*R1 = 12V  (.3mA)(10k) = 12V  3V = 9V
Then from (3), Vg < Vd + Vt
Vg < 9 V + .7 V = 9.7 V
So Vg, the gate voltage of U4, must be less than 9.7 V.
I choose 6.7 V for the gate voltage to get well below the edge of saturation.
This is what I have so far. I then attempt to do a DC sweep analysis to find the transfer characteristic of (VO2VO1) vs (VINP  VINN). Once I have the transfer characteristic, I will find the maximum linear region and compute the DC gain.
My problem is that I am not sure my procedure for doing the DC sweep of a differential amplifier is correct. I have attempted to use a parameter Vbias (as can be seen in the circuit) to sweep both input offset voltages simultaneously. The waveform I get though after placing a probe at one of the amplifier's outputs shows that the input offset bias should be around 3.5 V (in the linear region), which was not what I calculated (~6.7 V).
Any help regarding whether my calculations are wrong or whether I am doing the DC sweep incorrectly to find the transfer characteristic would be greatly appreciated.
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