# How to solve the (b) circuit?

In circuit b there is a Zener, LED and transistor the voltage of base I assume is 12 V - 6.2 V = 5.8 V so I guess Ve is Vb - Vbe = 5.8 V - 0.7 V = 5.1 V but I can't guess the current which is passing the LED. Then can I guess the voltage of 200 Ω?

Example: 6.2 - 0.7 = 5.5.

How can I solve it?

• You're well on your way! The voltage across the 200Ω resistor would indeed be 5.5 Volts. To solve the question further, however, you would need to have a beta (or gain) value for the pnp transistor. Does your question provide any more information? Commented Dec 8, 2019 at 8:47
• Is your Ve calculation correct? This is a pnp bipolar.
– Chu
Commented Dec 8, 2019 at 8:48
• Well beta is given 100 and is there any problem at Ve? Commented Dec 8, 2019 at 8:51
• Is the voltage of Vbe 0.7V? Commented Dec 8, 2019 at 8:59
• No..............
– Chu
Commented Dec 8, 2019 at 9:00

From the question:

Can I guess the voltage of 200 Ω? (Example, 6.2 - 0.7 = 5.5).

This is correct. From that you can work out the current through the 200 Ω resistor and when you have the β value you can work out the LED current.