Transistors matching for double input stage (symmetrical)

[vitt]

Hi All,

Reading this forum and great DSelf book I decided to match the input stage transistors in my car amp.

It has pretty standard schematics: double (symmetrical) diff. amp, resistive tail current; no current mirrors, just a resistor in one leg. Loaded by symmetrical VAS. Each half contains only one transistor and degeneration resistor in emitter.

As I can see the common agreement here is to match transistors "in circuit", but matching 4 transistors at the same time may be difficult? While we can easily monitor/compare Vbe, the Hfe might be "masked out" by the fact that current flew directly between the bases.

Ideally I would like to see voltage offset between bases 2mV or less. It will allow me to get rid of NFB capacitor (I decrease amp. gain to 10).

Also, matching legs currents would be good addition. But I'm not sure if this possible for this schematics.

All impedance related DC offset "eliminated" by matching emitter degeneration resistors and impedances seen by bases. Got 25/25 of 2SA970/2SC2240 waiting to be installed...

Thanks for any ideas!

 

[lineup]

Is it 12 Volt supply?
Good you have 25 pieces of transistors.

You should find a good matched pair of SC2240.
And a good matched pair of SA970.
Then use them.

To try and find 4 matched of two NPN and two PNP will not happen.
NPN and PNP have different HFE and different VBE.
This means that there will be more base current for one pair
and less for the other.
This creates what is called one Input Offset Current
and this current creates a voltage drop across input resistance
and this gives Offset Voltage which will be multplied with gain of amp.

It might be that you therefor must keep the NFB capacitor.
To make the Input Offset Current very small
it is not unusual to run input transistors at low current, like 0.150 mA.
This of course means only a few uA needs to bias via bases.

JFET input stages does not have this problem.
Because they need almost no bias current at all.

 

[vitt]

Is it 12 Volt supply?

No, +-30V.

You should find a good matched pair of SC2240.
And a good matched pair of SA970.
Then use them.

Yes, but does it mean I should match them with DDM or similar (as opposed to "in-circuit)?

To try and find 4 matched of two NPN and two PNP will not happen.
NPN and PNP have different HFE and different VBE.
This means that there will be more base current for one pair
and less for the other.
This creates what is called one Input Offset Current
and this current creates a voltage drop across input resistance
and this gives Offset Voltage which will be multplied with gain of amp.

No. I think if we have perfect match for NPN (say Hfe=200, Vbe=0.65) and perfect match for PNP (say Hfe=150, Vbe=0.62) we will see no DC offset. Because current for non-inverting input will be equal to current for inverting ("Ibnpn - ibpnp" for both).

To make the Input Offset Current very small
it is not unusual to run input transistors at low current, like 0.150 mA.

Interesting: I wanted to _increase_ current (now it 2mA, want 4mA). Low current mean low transconductance (why they call it this way?)