Input stage biasing - BJT

[AndrewT]

Hi,
yes the BL is the higher gain and expect about 350 to 450 with only a few getting into the higher hFE.

Some builders suggest matching hFE but I disagree, I consider it much more important to match Vbe, but ensure that Ic or Ie is held at the desired operating level when taking the test measurements. Do the test at a low voltage to minimise heating of the junction. This unfortunately does not mimic operating conditions but it's the best we amateurs can do short of the pulse testing in the manufacturers' labs.

The exception for hFE match is the LTP pair and if possible the complementary LTP dual pairs. But even here the Vbe is the more important parameter.

[anatech]

Hi Steve,
This is where Andrew and I completely disagree. However I will point out that Vbe is normally close with matched pairs anyway. If your parts came from the same lot it should be easy to match both ways. DC beta (gain) is much easier to measure than Vbe due to the temperature affecting that reading very strongly.

I measure hFE at different temperatures and its reasonably constant. When I test diff pairs, I test them at the voltage and current levels I expect them to run at in the circuit. All you have to do then is not worry about the temperature (because they will operate at this higher temperature) and let them settle down. A test jig is easy to build for this purpose.

Hi Andrew,

The exception for hFE match is the LTP pair and if possible the complementary LTP dual pairs. But even here the Vbe is the more important parameter.

I guess this is where we agree more. NPN and PNP Vbe is always totally different, so matching them would be impossible. I'm just trying to balance the viewpoint. Not to challenge you.

Cheers, -Chris

[myhrrhleine]

Hello,

my $0.02

Hfe matching is important to reduce DC offset at the output of the amp.
Not an issue for you (IMO) since you are using c103 for that.
Q108 is used as a high impedence load.
Q109 is basically a diode driving Q108
Any current through D109 drives Q108 equally (mirroring the current in D109 to Q108)
The closer the diode matches Q108 the better.
Using a transistor for the diode (Q109) is even better.
The better Q108 and 109 match, the better.
Adding Q107 boosts the current to 109's base, meaning less current imbalance from the Q109 side making the balance even better.
Matching Q108/Q109 is important.
Also matching their emitter resistors is important.


R105 and 106 only need to be 20-200 ohms or so.
Personally, I like 100 ohms.

Q104 and Q105 are used to provide gain/local feedback to Q103/Q106 respectively.
A good balance of Q108/Q109 would get distortion so low that Q104/Q105 might not even be needed! It might be overkill on the input stage.

[steve_mak]

Hi guys,

I've sat down and redesigned the schematic ...

(I'm getting shy about paying more than $100 for another board though...)

I've added series resistors where total resistance is important.

I think I'll try to get a matched pair for Q103/Q105 and Q108/Q109.

All resistors are 1% 1206 surface mount 0.25W.

Otherwise, I think I'm getting close!

(Those large polyprop caps cost a bomb!!)

Cheers,

[CBS240]

Hi

R107/109 could probably be despensed of, they are too large for degeneration resistors, the 68 Ohms are more like degeneration resistor values. Your absolutely right about those polypropylene caps, the one I will be using for DC blocking costs $16 a piece. A comparable electrolytic is about 40 cents.


As for PCB's, I find it a lot cheaper to use Radio Shack cheap-O veroboard, solder bridge, and jumper wire the parts together to construct the first prototype just to get it to work before having the PCB made. It is easier to change around, and if you compleatly screw it up, your only out a few bucks.

[anatech]

Hi CBS240,
True enough! You even end up with a close layout to the final circuit.

-Chris

[steve_mak]

Hi CBS240 et al,

The R107/R109 resistors are shown as 3.3 ohms (I write it as 3R3 because the dot can get missed !)

All I'm trying to imply with them in the diagram is that the combination of R107+R108 and R109+R110 is important to be matched evenly.

The other thing is that WHEN (that's more positive! - better than 'if') a new pcb is made it is better to have a few spots where one can break a circuit for testing purposes ... an example might be R106 because I think although I've seen it in some circuits it can be zero ohms too. If I take it off the board I can test me current source independently.

Cheers,

[anatech]

Hi Steve,
Air wire. Once it's working you can decide if it's worth a new PCB.

-Chris

[CBS240]

Pardon the mis-undertanding, you are on the right track leaving extra parts available on the PCB for testing, or just in case you need them. Worst is that you have to short the connection or leave it out entirly. There are extra parts on the PCB that is in the mail now. Same idea. This leaves me the option of testing a few different types of compensation with the same circuit.

[steve_mak]

Yes, I do need to do a prototype but remember I'm using surface mount components!

I'll try to get excited about burning a board myself.

It involves technology I do not possess! (Like a printer...!)

:-)