I got an assignment to
1) Calculate open loop gain of the existing design of the amplifier
2) Configure this op-amp with the closed loop of 26 dB. The chosen resistors should not exceed 100 koHm
INPUT STAGE: Q111 and Q112
Av1 = Rtot/2re, where Rtot is the total resistance of R117 and the input resistance of Q78 which is Rin.
So the gain Av = Rtot/r'e. r'e = 250 oHms. If hFE for Q78 is 110, Rin = hFE*(R115+re) = 110(10.6 + 150) = 17666 ohms. So Rtot = R117||Rin = 15k||17666 = 8112 ohms. Using this value the gain Av1 = 8112/500 = 16.224
If the load is 2 kohms hFE Q94 is 40 (power transistor). Load impedance RL= 40 * 2000 = 80 kohms.
SECOND STAGE: Q78
Output resistance of Q78: Using Early voltage of 52V, Ro = (VA + VCE)/IC. ro = 26126 ohms. (Ic = 2.85 mA, Vce = 22.46 V)
damping factor = (r'e + Re)/r'e = (10.6 + 150)/10.6 = 15.15
Rout = ro * damping factor = 395809 ohms.
The early effect resistance and and load impedance are parallel. So Rtot2 = RL|| Ro = 80000 || 395809 = 66550
Gains at stage 2: G2 = Rtot/(R153 + r'e) = 66550/(150 + 10.6) = 414.4
OUTPUT STAGE: Q94 and Q126
For Q94, IE = 1.1mA So re = VT/IE = 25mV/1.1mA = 22.73 ohms
Rtot3 = R174 + r'e = 68 ohms + 22.73 ohms = 90.73 ohms
Both halves are in parallel, So
Output Impedance Zout = 90.73 ohms || 90.73 ohms = 45.37 ohms
Gain in the output stage G3 = RL/(RL + Zout) = 2000 ohms/(2000 ohms + 45.37 ohms)= 0.978
Thus open loop gain G = G1*G2*G3 = (16.224)(414.4)(0.978) = 6575
Converting to dB : Gain in dB = 20log(6575) = 76.36 dB
I have calculated the open loop gain considering a load of 2000 ohms. I ended up with an open loop gain of about 6575 which is about 76.36dB.
If the first part I was able to solve - Acl = 88 dB an Aol = 76.36 dB. I got stuck in the second one. Any thoughts, suggestions which way I should go or how to adjust the resistors in order to get the desired Closed Loop Gain result?
Thank you in advance!!!
SIMULATION with Rf and R2:
V+ = 10 mV (sine), V- = 10 mV Picture shows Vout
Resistor R39 = 99 Kohm, R40 = 301 ohm
6575 (open loop gain) / 20 = 329 should attenuation factor
The feedback then is: Av = 1 + R39/R40 = 329
The feedback should attenuate the signal by 329
But in this case does not really work