Is horrible.
I'm just debugging a nice diode laser controller for one customer and getting ready to do another one for another (much better) customer. It uses a ZXTP25020 PNP with a biggish degeneration resistor and two-pole bypassing of the base to get low noise at high frequency.
Turns out the 1/f noise is atrocious. The total noise from 2 Hz on up is only a couple of PPM, but it should be a factor of 10 better than that. With a total base resistance of 1k, the 1/f corner at 40 mA I_C is over 10 kHz.
The bias network looks like this:
+9V 0----------*--------*-------* | | | | | | | | R | 4.7 | 3.3 R 39 ohm TF | nF | nF R CCC CCC | CCC CCC | | | | / | | |< ZXTP25020 From 0--RRRR--*--RRRR--*---- | Op Amp |\ 499 499 | \ | | --- \ / --> V --> ----- | | GNDThe emitter resistor can drop up to about 5V.
I've got some higher-beta transistors on order (FZT788B), and will reduce the impedance of the bias network, but I was fairly shocked that the 1/f noise was so bad.
Cheers
Phil Hobbs