Recently I watched this EEVblog episode, about multimeter input protection:
Since that, I tried to figure out, what's the purpose of Q1 and Q2 transistors. (on page 58 in the service manual)
Some kind of voltage/current limiter? I also tried to do a simulation in LTspice, but I still don't have a clue.
If you have any ideas, please share them!
In case you will use 2N3904 the voltage will be somewhere between 8.8V-9.2V. The upper 2N3904 behaves more like zener diode ( 8.1V-8.5V) and lower like diode (0.7V).
For exact voltage you will needs to measure exact transistors.
Base emitter reverse breakdown, typically in the region of 6-10V. They are used because the junction has low capacitance and very low reverse leakage. Important in a high impedance circuit.
This is an old topic, & above two posters have reasonably answered "dobszi14"'s question.
But let me fill in the blanks & add some more meat into the answer for the benefit of other searchers.
Q1 & Q2 used here with their base & collector shorted, is effectively a back-to-back bipolar zener-diode, used for clamping/limiting a bi-polar input signal.
This has several advantages over standard back-to-back zener-diodes.
- Less Capacitance than a standard zener-diode as pointed out by Sean.
- Faster response/switching time vs zener-diode [ns vs μs/ms] (a zener switching is very slow)
- Knee (Vi-characteristic) of this configuration is sharper than the reverse-bias V-I charac of a zener. So before breakdown happens (input-voltage < limit-value condition), the leakage current is very low (neglible pa/nA). So it does not create inaccuracy in μA current measurement by stealing some of the "current-shunt resistor" current, when used in such configuration.
This transistor configuration is non-standard, and is not usually explained in most standard electronics text-books, so when you comes across it, for the first time, it puzzles & surprises you, as me. I encountered such a configuration first in the feedback circuit of a precision amplitude sinewave generator,in an avionics circuit, in my work. it was using BJT 2N2369. It was having a VEBO spec of 4.5V, but when reverse biased, with a current-limiting resistor, it always breakdown at 6.9V, acting as a fast zener. I initially thought this was a queer behaviour of 2N2369 alone.
I found it difficult to find a similar circuit behavior referred or explained in any electronics textbooks. But after some research & testing found that this was standard behavior of any BJT-transistor. But the breakdown voltage value may not be same, & is influenced by the doping of the base-emitter junction. But it will be around 6V.
Bear in mind, this is a non-standard use of a BJT. The breakdown process is avalanche breakdown effect, not zener break down effect, accounting for faster switching time, I think. And also the current-limited avalanche breakdown, does not damage or degrade the BJT-device, which is difficult to comprehend initially.
So as long u are not very specific about the value of the breakdown voltage to millivolts or 100's of millivolts-level, then you can safely use such a configuration to replace a zener.
Hope this helps others.