Buffer impedance

[spoontex]

Hello,

I've a lot of OC1045 GE transistors, with a very low leakage and low gain ( 20 - 30 hfe ). I've searched what to do with these transistors, and I found an article from AMZ that explains what to do with high leakage transistors.

http://www.muzique.com/tech/ge_buff.htm

But I think I need to rise R4 with the OC1045.

Zin = ( B + 1 ) x R4

Supposly that I have some AC128 with 100 hfe:

Zin = ( 100 +1 ) x 10000 =  1M

but for the OC1045 with 20 hfe:

Zin = ( 20 +1 ) x 10000 = 210K

It's a bit low impedance for a buffer no?

I'm wrong?

please correct me if it's wrong.

Anybody help me to calculate the Zout?

Thanks.

[PRR]

> a lot of Ge transistors, ...low gain ( 20 - 30 hfe ).

Darlington.

> Anybody help me to calculate the Zout?

Shockley's Law.

However here it is more about the 1uFd series cap. Transistor is ~~100r but the cap is >100r over most of the audio band. Also the 10k pull-up resistor won't like <30k load.

[antonis]

Supposly that I have some AC128 with 100 hfe:
Zin = ( 100 +1 ) x 10000 =  1M
but for the OC1045 with 20 hfe:
Zin = ( 20 +1 ) x 10000 = 210K
It's a bit low impedance for a buffer no?
I'm wrong?

It depends on signal source impedance..
(a 10% or lower signal atttenuation should be considered OK - this stands for signal source impedance of 20k or lower..)

BUT
your Zin calculation is lacking 'cause you didn't take into account voltage divider bias resistors & pull-down resistor (the last can be safely ignored due to its high value..)
So, for h_FE=20, Zin = [(20+1) X 10k] // R2 // R3 => 72k
Of course, you can raise bias resistors impedance by bootstrapping them but you can't do anything for raising h_FE, other than Paul suggested..
 

Anybody help me to calculate the Zout?

Again, what Paul told you..
I don't know how "low" is your OC1045 leakage so I can't safely tell you to just consider r_e (Emitter intrinsic resistor, 0.026/I_collector) as Zout...