# Another Failed Differential Amplifier

This is the circuit I made - designed it, calculated it, built it:

simulate this circuit – Schematic created using CircuitLab

Q1's and Q2's collector current was 5mA, while Q3's was 1mA. Sine wave at the input had 1Vpp at 1kHz. Negative feedback should work since there is a 360 degree shift between input at base of Q1 and base of Q2. Rf2 was firstly decided to be 10k, then it was replaced by potentiometer.

This circuit didn't work as I expected. I expected that if some distortion would occur inside sine wave then it would be corrected by negative feedback or/and differential transistor pair, and the amount distortion being corrected would be controlled with Rf2 (less gain - less distortion).

I made the distortion by adding another sine wave (1Vpp, 3kHz) to the base of Q3. The actual results couldn't be compared to the desired ones as they weren't even close to the desired ones.

As the result, the output at collector of Q3 was distorted in the same way as the signal at the base of Q3 - should there be pure sine at collector of Q3? But then I scoped the signal at the collector of Q2 and only there was the sine wave I expected to be at the output of amplifier (under the condition, that base of Q2 was shorted to C1, otherwise with rotating the potentiometer Rf2, the signal would rapidly approach the distorted one).

Sine wave at the collector of Q2 versus distorted signal at the base of Q3 (not on the same voltage scale).

I think that there is still a little gap in my understanding of differential amplifier because I am struggling with this for a while and I haven't made one useful circuit including diff. amp.

• How did you "add" a sine wave to the base of Q3?
– τεκ
Commented Dec 28, 2017 at 1:18
• @τεκ With another channel of my function generator via capacitor Commented Dec 28, 2017 at 1:23
• @Keno You are pretty close, really. You just didn't account for giving it the "room" in order for the NFB to work correctly at DC. So the added AC just can't work, either. I am seriously glad to see that you are putting things together and testing your thinking!!
– jonk
Commented Dec 28, 2017 at 1:29
• In order to reduce harmonic distortion, There must be much more open loop gain more than closed loop gain. Your Rc/Re open loop gains are too low here, so your negative feedback ratio of Rf2/Rf1 is low as well. Commented Dec 28, 2017 at 1:42
• @Keno I am very excited to see the recent two posts, though. Each is breaking up the work ahead into logical parts! Nice. (That's an advancement I think I've seen in you.) And no, it's not going to be easy to get all the details right. There are a lot of details. But you will learn so much from the process. I'll bet you'll be teaching me a few things, soon enough! Keep at it.
– jonk
Commented Dec 28, 2017 at 19:10

Sorry for misanalyzing the circuit - you actually have plenty of open-loop gain - about 100.

simulate this circuit – Schematic created using CircuitLab

(see discussion below)

The small signal resistance looking from the bases of Q1 Q2 is very different. I've made Q2's small by adding that a capacitor from the output to Vn. I'm using 10kHz as the "distortion" source since it's easier to see the wigglies.

Here it is without that capacitor

• I will go and check if your corrections make any difference but this shouldn't be a problem since I designed the circuit so that should base current through Rb and Rf1 be about 16 uA and 2V drop across it. Both Q1 and Q2 have beta of approx. 300, so 120k resistor for both bases is just right, don't you think? Commented Dec 28, 2017 at 15:22
• No, your addition of base resistors makes things even worse.. Commented Dec 28, 2017 at 15:29
• Those 120k resistors are in different positions though - Rf1 is in series with the base while Rb is in parallel. As an experiment try making Rf1 zero.
– τεκ
Commented Dec 28, 2017 at 15:38
• or putting a 1uF capacitor across it
– τεκ
Commented Dec 28, 2017 at 16:02
• No, that doesn't improve anything. The problem is not in base curents since voltage drops across both of Rc differ only for 0.5 V. Commented Dec 28, 2017 at 16:05

Your diffpair gain will be Rcollector / (2 * reac) = Rcollector * gm/2

Thus diffpair gain is 1,500 ohms / ( 2 * 5 ohms) = 1,500 / 10 = 150x.