# BJT amplifier question

#### bumclouds

Joined May 18, 2008
81
Hey guys.

I have an assignment which presents to me a basic BJT amplifer circuit that looks similar to this:

I've drawn it up in a circuit simulator, and no matter how I play with it, it is always inverting. Is this correct? I have fiddled with the VCC and VEE values, as well as all the resistor values.

Here's my problem though. My teacher has asked me to design an amplifier using this configuration to have a gain of greater than or equal to 30db. But how can you have this if the output is always negative? Doesn't this mean in your gain calculations it's always going to be negative?

I'm stuck

Regards,
Andrew

#### Audioguru

Joined Dec 20, 2007
11,251
The amplifier is inverting. Its output phase is inverted from its input phase.
Its output is always a positive voltage and is never a negative voltage.

Gain is how many times the output signal level is than the input signal level.
A gain of 30dB is about 33 times. The inverted or not phase has nothing to do with gain.
Maybe you could call attenuation negative gain.

Do you know what controls the amount of gain of this transistor circuit?

#### bertus

Joined Apr 5, 2008
19,944
Hello,

When you use a common-emitter amplifier the signal will aways be inverted.
For the gain calculation you do not need the - sign for the phase.
A negative gain will be an attenuator and not an amplifier.

http://users.ece.gatech.edu/~mleach/ece3050/index.html
There you will find the following PDF's
http://users.ece.gatech.edu/~mleach/ece3050/notes/bjt/CEAmp.pdf
http://users.ece.gatech.edu/~mleach/ece3050/sp04/CEAmpSp04.pdf

Greetings,
Bertus

#### bumclouds

Joined May 18, 2008
81
Do you know what controls the amount of gain of this transistor circuit?
I'm thinking that it is the ratio of RL (collector resistor) and RE? I'm thinking that because the NPN transistor sort of 'sucks' current through C and out E, and then you can use the voltage divider rule?

I'm not sure >_<.

EDIT: Actually I have tried altering RC and R1 (from diagram) and they both seem to increase gain?

Last edited:

#### Audioguru

Joined Dec 20, 2007
11,251
I'm thinking that the gain is the ratio of RL (collector resistor) and RE?
No because RE is bypassed by a capacitor which makes RE an extremely low impedance.
The gain is RL/Re. Re is the internal emitter resistance.
Sure the gain will change if you change the value of its resistors.