# Identifying the configuration of transisitors

I have been handed the diagram below, as part of an introduction to transistors. I am having a hard time finding out what type of configuration it is, therefore making it even harder for me to find the voltages and current values.

My intuition says the base needs to have some power source.

My question lies in identifying the configuration of the transistor [Common: {Base,Emitter,Collector}]

It's worth noting I have a limited understanding of transistor circuits.

EDIT: Would the circuit below , be a somewhat equivalent to the one above in terms of calculations, I am new to circuits especially ones that don't have the cells in them. simulate this circuit – Schematic created using CircuitLab

• How much current is flowing from ground to -20 V rail via the base? That should get you started. – winny Dec 20 '17 at 21:06
• Just as a thought experiment, what if you relabel -20 as ground, ground as +20, and +20 as +40. Does that schematic make more sense to you? And if so, what is the difference between the two? – WhatRoughBeast Dec 21 '17 at 1:49
• @WhatRoughBeast does the edit above(The schematic) coincide with your explanation. I think there is no difference between the two, hope am not wrong. – Bob Kimani Dec 21 '17 at 11:28

My intuition says the base needs to have some power source.

At first glance it does look that way, however, you have to remember that ground is really just another voltage level. If you look more closely at your circuit you will see that..

Since the base is connected to a higher voltage than the emitter.. the base DOES have a power source!

My question lies in identifying the configuration of the transistor [Common: {Base,Emitter,Collector}]

Does it matter for this problem?

In reality, since there is no indication of an output in the schematic, or the question parts, it is not possible to tell which one it is. The best you can state is "It is probably not common base."

But it is not relevant.

The math required to answer the problem parts is the same regardless.

Based on your limited understanding of transistors it will be hard to teach you the basics here. You'll need to read a book or some tutorials. There are a great deal of articles which you could find on the web. But I can give you a couple of simple facts by which you can answer all three questions by simply finding the collector current. There are two things to take into account.

1. If the (NPN) transistor is in active region the base-emitter junction would be forward biased, dropping 0.7V from base to emitter (i.e. base 0.7V more positive than the emitter).
2. The transistor current gain relates the base current to the collector current by the formula $I_C=\beta I_B$. Since $I_E=I_B+I_C$, the emitter current becomes $I_E=(\beta+1)I_B \approx I_C$ - since $\beta$ is typically large. This is also true if the transistor is operating in the active region.

Once you'll find the emitter current, you can easily calculate the emitter, base, and collector voltages. To get started, just use KVL around the loop containing the base and emitter. This will allow you to find the emitter current, which according to the fact # 2, is almost the same as the collector current.

Regarding your confusion about which configuration the circuit is. We can't tell unless you specify where the output voltage of the circuit is sensed. If the output is sensed at the collector it's a common emitter (here with emitter degeneration). If the output is sensed at the emitter it'll be a common collector.