Quote:
OK. Q5 and Q6 are not a current mirror because the collector voltages are different and their currents are not equal to each other, but the collector voltages don't change, neither do the base or emitter voltages, therefore, they are both constant current devices, just, one of them carries more current than the other.

I would not have said Q5 and Q6 do not make a current mirror. The fact that there are small differneces in current due to different collector voltages ( and other causes ) doesn't disqualify the configuration as a current mirror. Neither does the way in which current is reflected from the diode connected device. Current mirrors come in different forms.

The transistors in question are part of the Wilson current mirror, however I would say they make a current mirror on their own. In fact, they make the mirror of the Wilson CM. Q7 is just an enhancement to the basic CM, and doesn't change the basic operation.

 

I would not have said Q5 and Q6 do not make a current mirror. The fact that there are small differneces in current due to different collector voltages ( and other causes ) doesn't disqualify the configuration as a current mirror. Neither does the way in which current is reflected from the diode connected device. Current mirrors come in different forms.

The transistors in question are part of the Wilson current mirror, however I would say they make a current mirror on their own. In fact, they make the mirror of the Wilson CM. Q7 is just an enhancement to the basic CM, and doesn't change the basic operation.

I agree (and have pointed out previously a couple of times) that Q5 and Q6 are a current mirror, but to understand the operation of the Wilson mirror, I do think it is important to recognize that Q7 isn't just an "enhancement" since this implies that you could remove Q7 and still have a functioning circuit. You can't. Because now the mirror is backwards. In a basic mirror, Q5 is the output transistor. In the Wilson mirror, it is the input transistor. That's more than just an enhancement, it is a fundamental difference.

 

Well, you can use it without Q7. But you have to pick the output off Q3's collector. From the perspective of the differential amp, it doesn't matter which transistor is diode connected. I guess we'll disagree ( once again ) the Q7 makes a fundamental difference.

 

Well, you can use it without Q7. But you have to pick the output off Q3's collector. From the perspective of the differential amp, it doesn't matter which transistor is diode connected. I guess we'll disagree ( once again ) the Q7 makes a fundamental difference.

Q7 raises the output impedance of the current mirror, and therefore the gain of the amplifier. It also reduces the amplifier's input offset voltage below what it would be with the basic current mirror. Add a 4th transistor to make it an improved Wilson current mirror, and the offset is even less.
I ran sims on the attached circuit. Input offset voltages were as follows:

V(out_wilson_improved):     9.53674e-006     voltage
V(out_wilson):     -0.000144005     voltage
V(out_basic):     0.000612259     voltage

Low-frequency gain for the amp with the basic current mirror was 72dB.
The two Wilson current mirrors each yielded 109dB.

Playing devil's advocate, 100+dB of open loop gain, with near-infinite output impedance, is difficult to take advantage of unless the next stage is a MOSFET. A similar situation exists for offset voltage. The bias current of the next stage will affect offset voltage.