Common Emitter Amplifier Design

Easy to use step by step guidelines for the design of a common emitter transistor amplifier stage showing calculations and the way component values are determined.

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The common emitter amplifier is widely used and it is a very easy circuit to design.

There are a few straightforward calculations which can be combined with a simple design flow to give a sure-fire result. It is quite easy to adopt preferred component values in the common emitter amplifier design.

There are several variations on the common emitter amplifier and these can easily be accommodated in the design. The most basic form of common emitter amplifier design is the simple logic buffer / output, consisting of a transistor and a couple of resistors. This can have a few extra components added to enable it to become an AC coupled amplifier with DC biasing and emitter bypass resistor.

Simple logic common emitter amplifier design

This very simple design for a logic buffer or common emitter amplifier design is about as simple as any design can be.

The circuit design shows the transistor with an input resistor and a collector resistor. The input resistor is used to limit the current flowing into the base, and the collector resistor is used to develop that voltage at the output.

When a logic high is seen at the input this causes current to flow through R1 and into the base. This causes the transistor to switch on. In turn the voltage on the collector falls to almost zero and all the voltage is developed across the resistor R1.

It can be seen that there is a phase inversion. For a high input voltage, the output is low, i.e. the commpn Circuit of a basic common emitter transistor amplifier - this version is often used with logic circuits as a simple switch.

A common emitter amplifier acting as a buffer for a logic IC is very easy to design.

Although not the only way to design the stage, the following step by step guide could be used.

1. Choose transistor:   The choice of the transistor, marked TR1 in the diagram will depend upon a number of factors:
• Power dissipation anticipated.
• Switching speed required - for switching applications choose a switching transistor, not another form of transistor with a high bandwidth, ft.
• Current gain required.
• Current capability required.
• Collector emitter voltage.
These can all be anticipated with sufficient accuracy before the start of the design. Once the design is completed, the figures should all be checked to ensure the transistor is adequate with the values chosen.
2. Calculate collector resistor:   The determination of the collector resistor, R2 is achieved by determining the current required to flow through the resistor. This will depend upon elements such as the current that the circuit needs to deliver. It may also be that an LED indicator is required in series with the collector resistor. The current should be determined to give the required light output. The value of the resistor can be determined using Ohms law, knowing the current flowing through the resistor and the voltage across it.
3. Determine base resistor value:   The base current is the collector current divided by the value of β or hfe which is virtually the same. Ensure that there is sufficient current drive to turn the transistor ON for the lowest values of β even at low temperatures where values for β will be lower. Care should be taken not to drive excessive current into the base as switching may take longer as a result because excess stored charge needs to be removed.
4. Re-evaluate initial assumptions:   Once the design has been competed it is necessary to re-evaluate some of the initial decisions and estimates in case the final design has changed anything.

Simple AC coupled common emitter amplifier design

The basic circuit for an AC coupled common emitter amplifier design is shown below. Circuit of a basic common emitter transistor amplifier with single base bias resistor

This circuit is not widely used because it is difficult to define the exact operating point of the circuit as a result of variations in the values of β encountered.

The step by step process shown below could be used:

1. Choose transistor:   The choice of transistor will depend on factors including the power dissipation anticipated, collector emitter voltage, bandwidth and the like.
2. Choose collector resistor:  The value of this should be chosen so that the collector sits at about half the supply rail for the required current. The resistance value can be determined simply using Ohms law. The current value should be chosen to give resistance / output impedance that will be acceptable for the following stage.
3. Choose base resistor:  Using the β figure for the transistor, determine the base current. Then using Ohms law and a knowledge of the supply voltage and the fact that the base will be 0.5V (for silicon) above ground, calculate the resistor.
4. calculate decoupling capacitors:  Using a knowledge of the input and output impedances, determine the value of capacitor to equal the impedance at the lowest frequency of use. (Xc = 2π f C where C is in Farads and the frequency is in Hz).
5. Revisit calculations:  Revisit all calculations and assumptions to ensure they are all still valid in the light of the way the circuit has developed.

Comprehensive AC coupled common emitter amplifier design

By incoporating a few extra components into the common emitter circuit design, it is possible to provide a better level of gain and also improved temperature stability at DC.

The common emitter amplifier design is relatively straightforward. The following design flow can be used as a basis.

1. Choose transistor:   As before, the transistor type should be chosen according to the anticipated performance requirements.
2. 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. This will define the resistor value using Ohms law.
3. Calculate the emitter resistor:   generally a voltage of around 1 volt or 10% of the rail value 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.
4. 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). If a range for β is specified, work on the cautious side.
5. 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.
6. Determine base resistor values:   Assume a current flowing through the chain R1 + R2 of around ten times that of the base current required. Then select the correct ratio of the resistors to provide the voltage required at the base.
7. Emitter bypass capacitor:   The gain of the circuit without a capacitor across the emitter resistor is approximately R3/R4. TO increase the gain for AC signals the emitter resistor bypass capacitor C3 is added. This should eb calculated to have a reactance equal to R4 at the lowest frequency of operation.
8. 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.
9. 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.
10. Re-evaluate assumptions:   In the light of the way the circuit has developed, re-assess any circuit assumptions to ensure they still hold valid. Aspects such as the transistor choice, current consumption values, etc.

It is possible to obtain a more defined gain for the stage for higher frequency signals by placing a resistor (R5) in series with C3. For low values of voltage gain, this can be determiend from the simple relationship Av = R3 / R5. Circuit of a basic common emitter transistor amplifier with additional emitter resistor in capacitor bypass path

With a little practice, the various stages in the common emitter transistor amplifier design become second nature and they can be undertaken very easily. The choice of transistor can also be made more easily. As mentioned above, it is very important to use a switching transistor for switching applications - even transistors with a high ft, or cut-off will not perform as well as a proper switching transistor.

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