# design common emitter amplifier with -20 gain, (working backwards)

Discussion in 'Homework Help' started by ninjaman, Jul 10, 2016.

1. ### ninjaman Thread Starter Member

May 18, 2013
307
1
Hello,
I have a question about designing a common emitter amplifier. I know the amp will invert the input so a gain of 20 is what I am looking for as whatever goes in, will come out inverted and multiplied by 20.
Part of my coursework mentions that a rule-of-thumb is that the collector resistor (Rc) over the emitter resistor (Re) will give the voltage gain.
So, example:
Rc = 5.6k,
Re = 1k,
5.6k / 1k = 5.6 Av
I am struggling with the coursework a little, it starts by selecting a supply voltage and choosing a quiescent current of 1mA. So, if the supply is 9 volts, the output would be half the supply, 4.5 volts. So, 4.5 / 0.001 = 4.5k or 4.7k (real value resistor). Then if the same current is going through the emitter and another rule of thumb is that the voltage across this resistor is 8% - 10% of the supply. So, 10% of 9 volts is 0.9 volts, find resistor by 0.9 / 0.001 = 900 or real value 910 ohms.
with 4.7k over 910 = 5.16
I want to work backwards. starting with 20 as a gain. if I keep the 9 volt supply and 1mA quiescent i will have 4.7k still. rearrange some stuff to get 20 / 4.7k to get 4.25 milliohms. thats a small resistor!!!?
I was hoping there was a way to do this. The instructions in my course are really basic and only show this one way.

thanks
simon

2. ### AlbertHall AAC Fanatic!

Jun 4, 2014
6,392
1,485
Try 4.7k / 20 => 220 ohms.

ninjaman likes this.

May 20, 2015
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