Transistors matching for double input stage (symmetrical) - diyAudio
Go Back   Home > Forums > Amplifiers > Solid State

Solid State Talk all about solid state amplification.

Please consider donating to help us continue to serve you.

Ads on/off / Custom Title / More PMs / More album space / Advanced printing & mass image saving
Reply
 
Thread Tools Search this Thread
Old 17th August 2010, 06:04 AM   #1
vitt is offline vitt  Russian Federation
diyAudio Member
 
Join Date: Feb 2010
Default Transistors matching for double input stage (symmetrical)

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!
  Reply With Quote
Old 17th August 2010, 07:18 PM   #2
lineup is offline lineup  Sweden
diyAudio Member
 
lineup's Avatar
 
Join Date: Dec 2005
Location: the north
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.
__________________
lineup
  Reply With Quote
Old 18th August 2010, 09:41 AM   #3
vitt is offline vitt  Russian Federation
diyAudio Member
 
Join Date: Feb 2010
Quote:
Originally Posted by lineup View Post
Is it 12 Volt supply?
No, +-30V.

Quote:
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)?

Quote:
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).

Quote:
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?)
  Reply With Quote

Reply


Hide this!Advertise here!
Thread Tools Search this Thread
Search this Thread:

Advanced Search

Posting Rules
You may not post new threads
You may not post replies
You may not post attachments
You may not edit your posts

BB code is On
Smilies are On
[IMG] code is On
HTML code is Off
Trackbacks are Off
Pingbacks are Off
Refbacks are Off


Similar Threads
Thread Thread Starter Forum Replies Last Post
Double transistors in the VAS Why? GEirin Solid State 7 25th February 2010 10:08 PM
Counterpoint - A Fully Symmetrical I/V stage Russ White Digital Source 22 26th October 2008 02:36 PM
Symmetrical input stage hjelm Solid State 19 22nd April 2004 01:37 PM
Matching Transistors Vivek Solid State 4 3rd February 2004 05:38 AM
jFET input ,full symmetrical class A circuit promitheus Solid State 28 20th November 2003 07:12 AM


New To Site? Need Help?

All times are GMT. The time now is 09:21 PM.


vBulletin Optimisation provided by vB Optimise (Pro) - vBulletin Mods & Addons Copyright © 2014 DragonByte Technologies Ltd.
Copyright 1999-2014 diyAudio

Content Relevant URLs by vBSEO 3.3.2