# Common emitter amplifier Electronics

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1. Mar 2, 2015

### Alexa431

Rb=40 ohms
RL=100 ohms
Vcc=15V

I need help in figuring this problem out, I understand if anyone here doesn't want to give me the answer but I would appreciate if someone would give me a formula in which I could solve the problem myself.

2. Mar 2, 2015

### LvW

Alexa - are you sure about the value of Rb=40 Ohms? This is equivalent to a base current in the order of 100mA.
Perhaps Rb=40kohms?

3. Mar 2, 2015

### Alexa431

Yeah Its 40 ohms I checked again to make sure.

4. Mar 2, 2015

### Staff: Mentor

This looks like a description of a lab exercise. Is this an experiment that you've carried out? What sort of help are you looking for?

Determining the gain theoretically will involve modelling the transistor to some extent.

5. Mar 2, 2015

### Staff: Mentor

I agree with LvW -- that circuit looks to have problems. Is this for a lab? Or are you supposed to simulate it? Have you calculated its quiescent operating point?

EDIT -- Dang it! Beaten out by gneill again!

6. Mar 2, 2015

### Alexa431

Well I was in charge of doing the theory part (formulas) and my partner the actual experiment if his numbers are wrong then, I would repeat the experiment myself but I just need to figure out how to solve the v0 on paper.

7. Mar 2, 2015

### Staff: Mentor

You'll need to know the frequency range that was used and the capacitor value (its impedance will be of the same order of magnitude as the circuit's resistors unless the frequency is pretty high). You'll also need to know something about the transistor used, either its specific part number or know its β or have its characteristic curves on hand.

8. Mar 2, 2015

### Alexa431

The capacitor was 0.1uF, the B was 100

9. Mar 2, 2015

### Alexa431

All in all I just want a standard formula which could be used to solve this circuit and circuits like these, all I need is the theory part of it

10. Mar 2, 2015

### Staff: Mentor

Well, with only the β you can start by finding the approximate DC operating point. You'll have to assume a typical value for the base-emitter voltage. Since the base current is going to be pretty substantial for this circuit (I'd guess on the order of a couple of milliamps thanks to the collector current pulling down the voltage at the collector and limiting the voltage at the "top" of RB), that base-emitter voltage will probably turn out to be somewhat higher than "typical".

11. Mar 2, 2015

### Staff: Mentor

Unless you can find this circuit already analyzed somewhere you won't find a "standard formula". There are many different configurations for transistors, each requiring their own analysis and producing different formulas. The approach here would be to replace the transistor with a simple equivalent model and then apply circuit analysis to the resulting circuit.

12. Mar 2, 2015

### rude man

The 'standard way' to analyze a circuit like this is the following:
1. assume infinite beta so base current = 0
2. therefore, the input current is Vin/Zc = current thru the feedback resistor Rb. Zc = 1/ωC.
3. this gives you Vc = collector voltage.
4. sum currents to zero at the collector. This gives you Ic = collector current.
You can use this approach for either ac or dc analysis.
Problem is, your resistor values are way too low to make this approach work in the lab. Who came up with 40 and 100 ohms? What is the range of ω? What is Vcc? I predict a puff of smoke for this circuit ...

13. Mar 3, 2015

### LvW

Yes - there is something like a "standard formula"; better: Standard procedure for finding the ioperational DC point:
You have to solve a system of two equations with two unknowns (assuming a base-emitter dc voltage Vbe=0.8V):

* Base current Ib=Ic/B=(Vc-0.8)/RB
*
Ic+Ib=Ic(1+B)=(Vcc-Vc)/RL

The two unknowns are the collector voltage Vc and the current Ic.
However, as mentioned already - this is a pure academical exercise without much practical relevance.

14. Mar 3, 2015

### LvW

Alexa, I have simulated the circuit using a power transistor - and it works!
However, because the voltage Vce is in the order of 1V only, the input signal must not exceed a value of app. 10mV.
This applies for a very large coupling capacitor (100uF) and a frequency which is sufficiently high (some kHz).