I'm facing something I cannot fully understand even though I tried for hours...
Under simulation (LTSpice and Orcad), it sometimes, under some circusntances, stops working (Q1/Q2 keep always ON, and so capacitor stops charging again, thus oscillation ends). I found it to be dependant on R1 value and Q1 hFE..
If Q1's hFE is below 200 it works for some R1 values that don't work for higher Q1's hFE...
It seems that this kind of circuit is not very well suited for simulations. Maybe it is not a neat design? Thanks for any idea.
Hi Elerion,
I wrote this response before most of the other posts came in, so it covers some of the points that the other members have already made.
(1)
General
Sorry to say this, but as you have found, this circuit has a few basic problems and to even work at all depends on component parameters- not a good thing. Having said that, though, it is quite clever and uses the minimum components. You are hoping for a multivibrator saw-tooth generator, but the circuit is basically a monostable and will stay in the low state indefinitely.
(2)
Initial Points
(2.1) As has already been noted, the constant current (Ik) from the constant current generator Q2 is way too low at around 5uA. At a current that low, transistor hFE and ft drop-off badly. Worse still, the low current will not be able to charge up parasitic and virtual capacitances very quickly. The end result is that the circuit will tend to be unpredictable.
(2.2) Not only is Ik to low, but it is ill defined and will depend greatly on Q2 junction temperature, hFE to an extent, and individual transistor VBEs (bulk resistance effects).
(2.3) The saw-tooth high and low points are not well defined and will also depend on temperature, Ik and individual transistor characteristics.
(2.4) When Q2 is turned off, Q1 base is open circuit, so the node Q2C/Q1B is a high impedance. Consequently, that node will be badly affected by stay pick-up and leakage currents, especially at high temperatures. Also, when Q2 turns off there is no clear discharge path for the capacitances. The net result is that Q1 will turn on fast but will take an age to turn off. Finally, with a high impedance node like that there is more chance of parasitic oscillations.
(3)
Solutions
The attached image shows your original circuit (CURCUIT #1) with a few voltages and currents indicated. CIRCUIT #2 shows the same circuit with some minor modifications, which should make the saw-tooth generator work- after a fashion. CIRCUIT 3 shows more extensive modifications which will, if I am right and there are no gross errors, work OK.
(3.1) CIRCUIT #1
There are two main problems with CIRCUIT #1:
(3.1.1) The very low and ill-defined Ik (note the very low voltage across R3).
(3.1.2) The mechanism for multivibrator operation relies on Ik not being sufficient to turn Q1 on. As Q1's base is not terminated this is impossible with an acceptable value of Ik, as discussed by Les and mikeA.
(3.1.3) Not a problem as such, but for development, it would be better to make C1 larger; increasing from 10nF to 100nF is recommended.
(3.2) CIRCUIT #2
(3.2.1) Ik has been increased to a much better 110uA, and is also reasonably well defined by changing the values of R1 and R3.
(3.2.2) A mechanism for multivibrator operation has been incorporated at the base of Q1. As a consequence Q1 base is also terminated. The principle is that when Ik flows through R6 the voltage drop across R6 will not be sufficient to turn Q1 on, so that the saw-tooth generator will not latch in the low state. But when Q2 conducts at the positive excursion of the saw-tooth, C1 discharge current plus Ik will be sufficient to turn Q1 on. Once C1 has discharge the current will drop back to Ik, so Q1 will turn off again. And so the cycle will continue- hopefully! You may need to adjust the value of R6 for optimum results.
(3.3) CIRCUIT #3
CIRCUIT #3 builds on CIRCUIT#2, buy adding a longer and better defined time for the saw-tooth low period, by virtue of C2 and associated resistors. I haven't bothered to work out the exact time, but you could experiment with different values of C2 to get the low period that you want.
(4)
Summary
I guess that you are investigating this approach for the experience rather than to build a saw-tooth generator per se. In case you don't already know, there are much easier and better performing ways. Hope this is of some use and you haven't been bored to much reading it.
Chuck
(EOE)
ERRATA
(1) '@= CHANGED COMPONENT' should read: '@= NEW COMPONENT'
(2) On CIRCUIT #1, label '700mV to 3V1' should attach to 'OUTPUT SAWTOOTH' not 'OUTPUT 0V'
(3) '4V16' in two locations should read '4V36'
(4) 'Ik=100uA' in two locations should read 'Ik=110uA'
(5) Q2-4= Q2-1, Q2-5= Q2-3, Q1-6= Q2-2