[time-nuts] Switching transistors, current sources, nonidealties and noise

Attila Kinali attila at kinali.ch
Fri Jul 1 07:56:30 EDT 2016


Thanks everyone for the answers!

On Mon, 20 Jun 2016 01:45:24 -0400
Charles Steinmetz <csteinmetz at yandex.com> wrote:

> The transation frequency of the current source transistor is part of the 
> cause, but the primary cause is generally the capacitance of the CS 
> output node to ground.  Some designers put an inductor in series with 
> the output, but I have never found this to be very effective [except in 
> poorly-designed simulations] due to the self-capacitance of the 
> inductor.  Much better, IME, is to add a cascode device to the current 
> source.  (See attached images.)  This has the added benefit of 
> increasing the output resistance.  This increase can be very substantial 
> (several orders of magnitude) if you use a FET cascode device as shown.

I simulated a couple of circuits, with very different results.
First thing that struck me was, that it is neigh impossible to
make cascode circuits stable when using RF transistors. And even
if I managed to do that, small changes in resistor values would
imediatly make it oscillate again, or degrade performance severely.
Same goes for using Darlington circuits (which I tried in order to
minimize the effects of beta variation).

The best results I got was with the attached circuit. Ie using
a classical opamp based npn current source, but using an emitter
follower between transistor and opamp in order to enhance high
frequency (aka transient) performance. R29 is there to load Q7
and to prevent it from going into saturation. R30 is needed for
stabilizing the circuit (I do not exactly understand what the
mechanics of the oscillations are, when R30 is removed, if someone
knows, please tell me). The voltage divider R30/R31 helps to keep
the opamp output away from the lower power rail. If stability is still
an issue, a 5-10pF capacitor should be added from the output of the
opamp to the inverting input (degrades frequency response below 1MHz slightly).

The simulation output shows the current through the (zero) voltage source
at the tail of the differential pair. The I(V11) curve is the circuit as
shown and the I(V7) curve is the same circuit with the two BFU520 replaced
by 2N3904. As can be seen, the transient of the 2N3904 is several times
larger than the one of the BFU520 and lasts for about three times as long.

I have not done any analysis of the temperature stability, yet.
My guess would be that is dominated by the input offset voltage
temperature coefficient of the opamp. But I have no calculations
to prove it.
Noise analysis would be interesting, but I doubt there is enough
data available to actually get some meaningfull results out of it.
> > Why do people use general purpose transistors in these places, even
> > though RF transistors definitly improve switching behaviour?
> > I dimply remember that someone said/wrote once, that RF transistors have
> > a higher noise. But if I look at the datasheet, the quoted noise figure
> > for the BFU520 is <1.6dB while the noise figure of the 2N3904 is 2dB best case.
> I, for one, have said this, but you are not remembering the whole point. 
>   RF transistors are generally considerably noisier AT BASEBAND than GP 
> transistors, both because their geometries are inherently noisier and 
> because they have *much* higher flicker noise corner frequencies 
> (usually 10kHz to some MHz for RF transistors, compared to 10Hz-1kHz for 
> GP transistors).  One might think that this would not matter at RF, but 
> the flicker noise modulates the bias of the transistor (and sometimes 
> other circuit elements), leading to both simple noise modulation as well 
> as phase modulation.  RF transistors are not specified for noise at 
> baseband.

Hmm.. if the flicker noise corner frequency would be in the few 10kHz to
100kHz range, then I would not be worried. The opamp's control loop
should "kill" anything below ~100kHz and dampen quite a bit up to 1-2MHz.
I would even suspect that in the <10kHz range, the noise of the opamp would
dominate the noise of the transistors.

> I modeled the ...01a circuit using a BFR90A BJT as the cascode device, 
> and the simulation showed that the current spikes were reduced by about 
> 50%.  However, my experience tells me that this would not hold in 
> practice.  

Do you know where the discrepancy between simulation and reality comes from?

			Attila Kinali

It is upon moral qualities that a society is ultimately founded. All 
the prosperity and technological sophistication in the world is of no 
use without that foundation.
                 -- Miss Matheson, The Diamond Age, Neil Stephenson
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