S
Steve Kavanagh
- Jan 1, 1970
- 0
I want to design some low noise bipolar amplifiers at HF (say 1-30
MHz,typically 5 MHz). But almost no data sheets contain info on
transistor noise performance at these frequencies. I would appreciate
any info on how to estimate/predict HF noise figure. Some specific
questions:
1. I could use low noise UHF/microwave transistors if I can keep them
stable. But usually they have no noise figure characterization below
200 MHz or so and complete noise parameters are usually given only
above 500 MHz or so. Is it practical to estimate HF performance from
this data ?
2. I have run across many data sheets for low noise transistors with
ft of a few GHz where the noise figure flattens out below 300 MHz or
so, which would appear to mean the behaviour is not much affected by
the device capacitances and I should be able to use the sort of
approximations typically used at audio frequencies. But some of these
also show (for a few mA Ic) the NF with optimum and 50 ohm source
impedance to be the same in the flat region, which doesn't seem to
make any sense to me. I would expect lower NF at higher source
impedance. What is going on here ?
3. How much can I trust manufacturers' SPICE models ? I've noticed a
tendency for RB to be a round number like 10 ohms which makes me
suspect that (a) the model is not intended to work well for noise
figure or (b) the unit-to-unit noise variation is too much for the
modellers to worry about getting an exact RB, or some combination of
these.
4. Have you any suggestions for commonly available devices which give
really low noise at HF (better than the venerable 2N5109, for
example), especially at low currents ?
Steve
Note: The email address above may not work...you can easily figure out
what will !
MHz,typically 5 MHz). But almost no data sheets contain info on
transistor noise performance at these frequencies. I would appreciate
any info on how to estimate/predict HF noise figure. Some specific
questions:
1. I could use low noise UHF/microwave transistors if I can keep them
stable. But usually they have no noise figure characterization below
200 MHz or so and complete noise parameters are usually given only
above 500 MHz or so. Is it practical to estimate HF performance from
this data ?
2. I have run across many data sheets for low noise transistors with
ft of a few GHz where the noise figure flattens out below 300 MHz or
so, which would appear to mean the behaviour is not much affected by
the device capacitances and I should be able to use the sort of
approximations typically used at audio frequencies. But some of these
also show (for a few mA Ic) the NF with optimum and 50 ohm source
impedance to be the same in the flat region, which doesn't seem to
make any sense to me. I would expect lower NF at higher source
impedance. What is going on here ?
3. How much can I trust manufacturers' SPICE models ? I've noticed a
tendency for RB to be a round number like 10 ohms which makes me
suspect that (a) the model is not intended to work well for noise
figure or (b) the unit-to-unit noise variation is too much for the
modellers to worry about getting an exact RB, or some combination of
these.
4. Have you any suggestions for commonly available devices which give
really low noise at HF (better than the venerable 2N5109, for
example), especially at low currents ?
Steve
Note: The email address above may not work...you can easily figure out
what will !