Hi

Im designing an common emitter amplifier and I have the following specification.

Frequency response:
Lower cut-off frequency: 40 Hz
Upper cut-off frequency: 1 MHz

I have worked out a suitable value for a coupling capacitor to give me that lower cut off but I have never covered how to implement an upper cut off frequency before?

I would know how to calculate the desired value fine but where in a common emitter amplifier would you place a capacitor to give you an upper cut off frequency?

Sorry if this isnt the right place fot this question, I know its a bit basic but im totally stuck.

Thanks for any help.

 

Do you have a SPICE program where you can simulate the amplifier on you computer?

Connect a capacitor from collector to ground. Adding capacitive loading will slow down the transistor.

Connect a capacitor across the collector resistor. Same effect.

Connect a capacitor from C to B. “Miller” capacitor will slow down the amplifier.

I believe your emitter is at ground. Break open the E to ground connection. Add an inductor. At low frequencies (below 1mhz) the inductor will look like a short. At high frequencies (above 1mhz) the inductor will add emitter resistance. The gain is the ratio of R-Collector and R-Emitter.

If you are really desperate, add RC low pass filter to the base.

I know, too many options!

 

by upper cuttoff frequency thats gonna be an LPF (series resistor then cap going to ground) You can add this on the output/collector. Guessing you can do the calculations for that already.

 

Thanks for your help guys,

Ive had to split my emitter resistor into 2 (bypassing one of them with my parallel capacitor to ground at my cut off frequency), to give me the midband gain i require and I think my circuit is sorted now.

One really stupid question though, this is my first ever circuit and i have never used multisim but i have to test to see if my characteristics of the circuit meet the spec in my textbook.

How would i measure to see if my gain and frequency responses work, i think i need to use the bode plotter but im not sure how i would connect it up?
Sorry for the newbie question but this work is a year ahead of where I am at (ive just started my course but i think i get the basics well.)

 

you could use a signal generator to your amplifier i/p. vary the frequency while using an oscilloscope to measure the o/p. When you just exceed the upper cut-off, your o/p should measure zero or nearly so. Likewise for the lower cut-off.

Hope this helps

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