[Argus25]

The whole matched transistor thing got me wondering, how well matched were transistors that were sold in pairs ? Luckily I had some to test , the results are interesting. I tested two lots of OC72's, that were matched by Mullard (see attached photo).

I set up a circuit to bias them into class A with 5mA emitter current.

( I'll describe the circuit in case anyone wants to try it, all resistors 2%)

The circuit was powered by 12V battery, the transistors had 1k2 collector resistors and 200R emitter resistors, bypassed by a 10uF electro. The base to ground (positive) resistor was 6k8, and the base to -12V bias resistor 51k. In series with each base was a 1k2 resistor and a 0.47uF coupling cap. This connected to a 1kHz signal generator, the ground of which was connected to +Ve.

This established an emitter current of just under 5mA

With 160mV pp from the generator's terminals the collector voltage swing was 6V pp.

When I tried the first "matched pair", with the same batch code, from Mullard's packet, the emitter voltages and currents were within about 1 to 2% of each other, And the output amplitude of the 1kHz sinewave practically identical within a few % at 6Vpp. Very difficult to tell the transistors apart, which I guess was the aim of the matching.

However, the supposed matched transistors from another white packet, which interestingly had different batch codes, where quite different at least 15% for the emitter voltages(currents) and dynamic gain. Made me wonder if this packet had been tampered with, as it was opened. And probably its less likely that transistors from different batches would match, but I guess they could.

A then tested a couple of random AC188's, these were over 15% different on both gain and emitter current, and the gain was about 20% higher than the OC72's.

So if at least one of these supposed matched transistor packets is anything to go by, Mullard looks like they did actually try to match them.

I do not have any more factory matched ones to test.