# Monostable Transistor Circuit not stable

I am just learning electronics, and bought a rather cheap oscilloscope (DS0138). Although it is not much, it is sufficient for my present needs. I am trying to use it to learn more about electronics.

I put together a Monostable Transistor Circuit It works just fine when R6 is 10k But when R6 is 100K, it is not stable. I have tried to figure out what is going on, but I cannot.

Any insight would be much appreciated. Thanks in advance.

• Which node are you measuring? – pipe Sep 30 '16 at 15:55
• Across the capacitor – Fed Oct 1 '16 at 13:04

R6 must supply enough current to saturate Q1 fully with all the current flowing through R4. It has to have less than about 0.5V Vce or Q2 will start to turn on, preferably more like 0.1V. So you should have about a 20:1 ratio of currents for most transistors (forced beta of 20).

The collector current of Q1 when on is about 5mA, and the current through R6 is about (5-0.6)/100K = 43uA (consider the capacitor an open circuit for this analysis). That's a ratio of more than 100:1.

Q1 probably has an hFE of well over 100 with a significant Vce, but the effective current gain drops as the transistor saturates and you need it to be well saturated or Q2 starts to turn on.

examine Rb/Rc ratio of Q1

when R6=100K, R6/ (R3//R4) = 100/0.5=200 , which is about the limit of the current gain of Q1.

When this happens the input impedance of Q1be is hFE *rbe is about the same as R6 and starts pulling it down as voltage increases on Q1-C causing R6 to become a negative ramp with positive feedback no longer saturating and becomes an astable (oscillator) at 100k.

• using an emitter follower on Q1C to drive the LED load would extend the range greatly.