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

Bob Camp kb8tq at n1k.org
Fri Jul 1 12:28:44 EDT 2016


> On Jul 1, 2016, at 7:56 AM, Attila Kinali <attila at kinali.ch> wrote:
> Moin,
> 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.

Real cascode circuits can be built with RF transistors. They also can be simulated.
Simulating them with the “standard” models is a PIA. The issue is that the inductance
of the package is not de-embedded from the test “socket” as carefully as it might be. 
There is also the somewhat non-intuitive need to stick a low value resistor in the base.
Done properly, they are very reproducible and reasonably insensitive to load. 


> 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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