Bypass Capacitor: What is its importance in an amplifier circuit?

October 15, 2021

In this blog, we will review the bypass capacitor, its function, and how it is important in an amplifier circuit. 

What is a Bypass Capacitor?

A bypass capacitor is added to an amplifier circuit in order to allow AC signals to bypass the emitter resistor. This effectively removes it from the output gain equation resulting in an increase to the amplifiers AC gain.

What is a common-emitter amplifier circuit?

The basic voltage divider bias common-emitter amplifier circuit consists of four resistors, and an NPN bipolar junction transistor. In addition to these core components, there are two coupling capacitors which effectively block the DC current from input signal as well as the output signal. The signal that appears at the Vin of the circuit is amplified, and phase shifted by 180 degrees when it reaches Vout.

Common Emitter Amplifier Circuit

We will go through an example with assigned values to better evaluate how great the AC voltage gain would be if a bypass capacitor was added to the circuit.


Bypass capacitor effects on a common-emitter amplifier circuit:

Step 1: To begin, we must calculate the base voltage, VB, of the circuit without the bypass capacitor. We must apply the voltage divider rule using the values RA, RB, and VCC.

VB Calculation


Step 2: Using the VB value obtained, we can then calculate the VE at the transistor. This is done by subtracting the base emitter voltage, VBE, from the calculated VB. A typical value of VBE is 0.7 V.

Calculate VE at the transistor


Step 3: The next step is to use Ohm’s Law to calculate the emitter current, IE using resistor RE, and VE.

IE Calculation


Step 4: Now we must calculate the internal emitter resistance, re, of the BJT. This value will vary with current and is approximated using a 25 mV voltage drop, the calculated IE, and using Ohm’s law.

re Calculation


Step 5: Calculate the voltage gain for the amplifier with, and without the bypass capacitor. Using a simplified relationship equation, we can calculate voltage gain without a bypass capacitor using the values we calculated.


For a circuit with a bypass capacitor, we will disregard resistor RE, as it is now bypassed for AC signals and does not contribute to the voltage gain. Using another simplified equation, we can calculate the voltage gain for the modified circuit.

Voltage gain without bypass capacitor

Voltage gain with bypass capacitor


Step 6: We can now compare the voltage gains calculated with or without the bypass capacitor by using a ratio of the two values.

Final Comparison
In conclusion, we can see that a bypass capacitor causes a significantly higher AC voltage gain in amplifier circuit 2, thus pulling more AC voltage than amplifier circuit 1.


We hope that this has been helpful as a student or practicing Electronics technician. If you have any questions regarding the Electronics Technician program, feel free to get in touch with us at info@gbctechtraining.com or give us a call at 1-888-553-5333 to speak with a Program Consultant.
 

Comments

Submitted by Gabriel Emmanuel (not verified) on Thu, 08/25/2022 - 10:31

Thanks for this it has been helpful to me

Submitted by Angel Garrido (not verified) on Thu, 03/02/2023 - 13:23

Then what to put Re at all ??. I mean , what to put a resistor at the emitter if it is going to be bypassed ?.

In reply to by Angel Garrido (not verified)

Submitted by Iris on Wed, 03/08/2023 - 15:15

The purpose of the bypass capacitor is to increase the AC voltage gain of the circuit. However, the DC voltage drop across RE is necessary for biasing, but any AC signal that is dropped across RE represents a direct loss in AC output voltage. Emitter Resistor (RE) is required to increase the DC stability of the circuit during DC analysis and for proper biasing. While during AC analysis, RE reduces the amplification, so we use the bypass capacitor to bypass its effect on the circuit. With RE bypassed the internal resistance of the transistor (re) is used in the calculation of the voltage gain, which yields the desired larger gain AV2.

Submitted by Manishimwe vallence (not verified) on Tue, 10/24/2023 - 00:08

Thanks for good notes rely it has helped me very much

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