Effect of Bypass Capacitor on Gain of Amplifier

 Here I wanted to record the effect of bypass capacitor used to bypass the flow of signal across the emitter resistor of an amplifier. Below is an example circuit.

This is multistage amplifier with two stages of common emitter BJT based amplifier with negative feedback from the second to the first. The bypass capacitor for the first and second stage amplifiers are C1 and C3 connected across the emitter resistors R3 and R7, respectively. The value of this capacitor should be chosen such that its reactance is less than 1/10 th of the resistances The BJT amplifier design calculator can in calculating the components values including the bypass capacitors and coupling capacitors.

🧮 Calculating the Bypass Capacitor:

The value of the bypass capacitor $C_e$ depends on the frequency response you want to maintain. We typically use the same method as for coupling capacitors:

Where:

• $X_C$ is the reactance at a given frequency (should be much smaller than the emitter resistor to be effective)

• $f$ is the frequency of interest (in this case, 1 kHz)

• $C$ is the capacitance value we need to calculate

🎯 Setting the Impedance Condition:

To effectively bypass the emitter resistor at 1 kHz, the reactance $X_C$ should be at least 10 times smaller than the emitter resistor $R_e$. So:

For $R_e = 180 \, \Omega$, we want:

Now, solve for $C$:

For $f = 1 \, \text{kHz}$ and $R_e = 180 \, \Omega$:

✅ Recommended Value: Typically 1 µF to 10 µF (Electrolytic or film capacitors) For 1st and 2nd stages, you could use 1–10 µF depending on desired performance. Usually, 1–4.7 µF is a good starting point.

🔄 Effect of Bypass Capacitors: Increase gain at high frequencies by bypassing $R_e$. Lower impedance at higher frequencies (allows more signal gain). No effect at low frequencies (as the capacitor acts like an open circuit).

I have used 2.2uF capacitor for C1 and C3. The following shows the input(200mV,1khz) and the output signal waveform of this negative feedback amplifier circuit.

Lets change the bypass capacitor to 0.1uF and see the effect of reducing the capacitor value than the suggested or calculated values. The following is the signal waveform.

 

 As can be seen the reduction of the value of the capacitor reduces the gain of the amplifier. Also there is phase shift of the input signal because it goes through two cascaded BJT amplifier circuit. I wrote about phase shifter circuit design with op-amp where users can set the phase shift between the input and the output signals.

 

 This circuit is a Non-Inverting Amplifier with BJT transistors which uses negative feedback amplifier design concept.