I am facing problem in solving such mixed circuits with bjt, diode, opamp and Zener diode.
In the above problem
1.zener can have 3 states (ON, OFF, Reverse ON)
2.diode can have 2 (ON, OFF)
3.BJT can have 3 (Cutoff, Active, Saturation)
Should I go for trial and error approach or is there a better method to approach such mixed circuit problem ?
BJT Circuit Problem – Troubleshooting Tips
bjtdiodessaturationzener
Best Answer
You can think of it this way, the Base of the transistor will be 0.7V above the emitter which is at -12V, so the base will be at -11.3V, the diode in series with the base adds another 0.7V bump, so the voltage at the right of the 1K resistor will be -10.6V, the Zener voltage is 5V and its anode is connected to the -12V supply, so the voltage at the cathode of the zener will be -7V.
Now you have the voltages at both sides of the 1K resistor, and since this resistor is in series with the base, the current through the resistor is the same as the base current, so the current is:
$$\frac{-7-(-10.6)}{1K}=3.6mA$$
The beta of the transistor is 30, so
$$I_C=\beta I_B=30(3.6mA)=108mA$$
Which is wrong, because that would imply that the colector voltage would be at -237V, so the current IC, is simply \$\frac{0-(-11.8)}{2.2k}=5.36mA\$ (I used -11.8 instead of -12 due to the 0.2V of Vce sat)