Frequency Response of Amplifiers In Practice

Audioguru

Joined Dec 20, 2007
11,248
An opamp has an output resistance of only 50 ohms to 75 ohms so it takes a lot of stray capacitance to reduce its high frequencies.

But here we have a transistor with an output resistance of 3000 ohms so a small amount of stray capacitance will cut its high frequencies.

A breadboard has more than a small amount of stray capacitance.
 

Adjuster

Joined Dec 26, 2010
2,148
If the amplifier has a gain of 100, then a feedback capacitance of 2pF can look like 200pF referred to the input.

If we accept Audioguru's 15pF estimate, it would come to more like 1500pF. Together with the 1000 ohm Rs, this would result in a lot of loss.

NB I see that your bias resistor Rb is 1MΩ. Depending on the collector current and current gain, this may result in an excessive voltage drop on the resistor, leading possibly to problems with biasing. For instance, at 1mA Ic and with β=100, the 1MΩ would drop 10V, which seems too big.
 

crutschow

Joined Mar 14, 2008
34,847
You can reduce the Miller Effect by reducing the 1kΩ input resistor value from the generator.

Also negative feedback will help. If you add an un-bypassed emitter resistor to reduce the gain, the Miller effect on frequency response is reduced proportional to the reduction in gain.
 
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