Hi
> On Jul 1, 2016, at 7:56 AM, Attila Kinali <att...@kinali.ch> wrote: > > Moin, > > Thanks everyone for the answers! > > On Mon, 20 Jun 2016 01:45:24 -0400 > Charles Steinmetz <csteinm...@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. Bob > 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 > <circuit.png><currents.png>_______________________________________________ > time-nuts mailing list -- time-nuts@febo.com > To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts > and follow the instructions there. _______________________________________________ time-nuts mailing list -- time-nuts@febo.com To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there.