How to increase bjt gain?

Choosing transistor

As before, the transistor type should be chosen according to the anticipated performance requirements.Iam using NPN transistor BC547.In order to have greater amplification you need to choose transistor with high β(Beta) Value.

The β value is also called as hfe value.You can see the hfe at the knob of multimeter.β is called amplification factor.With high β value,the transistor can be turned ON with low base current.We can easily determine the β(Beta) value using multimeter.

Calculate collector resistor

It is necessary to determine the current flow required to adequately drive the following stage. Knowing the current flow required in the resistor, choose a collector voltage of around half the supply voltage to enable equal excursions of the signal up and down.You can choose collector current at your own to have high current at the output,but the collector current should be within the supply current.This will define the resistor value using Ohms law.

For Example

To find out collector resistor

I had used the supply voltage and current as 15V and 1A. I need to have 0.5A current at output that is at collector.The voltage need to be half the supply voltage.So finally V=7.5V and I=0.5A(500mA).Using Ohms law

V=IR

7.5=0.5xR

R=7.5/0.5

The collector resistor value is R=15Ω.

Calculate the emitter resistor

Generally a voltage of around 1 volt or 10% of the supply voltage is chosen for the emitter voltage. This gives a good level of DC stability to the circuit. Calculate the resistance from a knowledge of the collector current (effectively the same as the emitter current) and the emitter voltage that is 10% of supply voltage.

For Example

To determine emitter resistor

I had used the supply voltage as 15V.So the emitter voltage should be 10% of supply voltage.The emitter current should be same as collector current.So finally V= 1.5V and I=0.5A.

Using Ohms Law V=IR

1.5=0.5xR

R=1.5/0.5

The emitter resistor is R=3Ω.

Determine base current:

It is possible to determine the base current by dividing the collector current by β (or hfe which is essentially the same).

For Example

The collector current is 0.5A.The β value is 300.

Base current =Collector Current/β

Base Current=0.5/300

Base current=0.0016A

The Base current is 1.6mA.

Determine the base voltage

This is easy to calculate because the base voltage is simply the emitter voltage plus the base emitter junction voltage. This is taken to be 0.6 volts for silicon and 0.2 volts for germanium transistors.

For Example

The emitter voltage is 1.5V.The transistor taken is silicon transistor that has 0.6V base emitter junction voltage.

Base voltage= emitter voltage + 0.6

Base voltage= 1.5 + 0.6

Base voltage= 2.1V

Determine base resistor

The voltage required at the base is 2.1V. It can be taken approximately as 2V. Choose the ratio of R1 and R2 resistors to provide the voltage required at the base.For choosing R1 and R2 resistor use voltage divider formula.The concept of voltage divider is clearly explained in STEP 4.

Vout=(VsxR2/R1+R2)

For Example

The Resistor R1 and R2 is connected between 15V and GND.So we need to use voltage divider formula to find out actual resistor value,so that we can get 2V at the base terminal of transistor.Supply voltage is Vs.Substitute the following value in voltage divider formula. R1=1KΩ,R2=160Ω,Vs=15V.

Vout=(VsxR2/R1+R2)

Vout=(15x160/1000+160)

Vout=(2400/1160)

Vout=2V

Thus we got exactly 2V at base using voltage divider formula.

Emitter bypass capacitor

The gain of the circuit without a capacitor across the emitter resistor is approximately R3/R4. To increase the gain of AC signals,the emitter resistor bypass capacitor C3 is added. This should be calculated to have a reactance equal to R4 at the lowest frequency of operation.The formula to calculate bypass capacitor C3 is given below.

C=1/(2πf)Xc

For example

Xc is emitter resistor(RE) value,that is 3Ω.

f is the frequency of AC signal to be amplified.Iam taking the frequency value as 95Mhz.That means iam going to amplify the AC signal that has 95Mhz frequency.

Substitute the following values in C=1/(2πf)Xc formula.

π=3.14,Xc=3Ω,f=95Mhz,Mhz=10^6.

C=1/(2πf)Xc

C=1/(2x3.14x95x10^6x3)

C=5.587216x10^-10

C=558.72x10^-12

C=559x10^-12

Thus we can take approximately as 600picofarad.

C=600pF.

Determine value of input capacitor value

The value of the input capacitor should equal the resistance of the input circuit at the lowest frequency to give a -3dB fall at this frequency. The total impedance of the circuit will be β times R3 plus any resistance external to the circuit, i.e. the source impedance. The external resistance is often ignored as this is likely to not to affect the circuit unduly.

The formula for calculating the input capacitor value is

C=(1/2πfR)

Where

R is the resistance of input circuit.The input circuit can be a oscillator or signal generator.

f is the frequency of AC signal to be amplified.

For Example

Let us take the value of resistance of input circuit R=500Ω for example.

Substitute R=500Ω.and f=95Mhz in the formula C=(1/2πfR).

C=(1/2x3.14x95x10^6x500)

C=3.350x10^-12

C=3.3pF.

The practical method of finding resistance of input circuit for calculating input capacitor is explained in STEP 5.

Determine output capacitor value

Again, the output capacitor is generally chosen to equal the circuit resistance at the lowest frequency of operation. The circuit resistance is the emitter follower output resistance plus the resistance of the load, i.e. the circuit following.

For Example

To calculate the emitter follower resistance,turn the multimeter to resistance mode.Connect the positive probe to collector terminal of BC547 transistor and connect the negative probe to ground where the emitter resistor is grounded.Note down the resistance value using multimeter.This is the method to find emitter follower resistance.

Let the resistance of the load be 1KΩ.Then the resistance to find output capacitor value is given below.

Resistance=Emitter follower resistance+Resistance of circuit following.

After obtaining the resistance value apply in the formula

C=1/2πfR