Transistor Common Emitter Amplifier | Electronics Notes
In the common base configuration, input is applied to the emitter and As emitter and collector current are almost equal, the collector Therefore we have a phase difference of degrees between the input and output. In electronics, a common collector amplifier is one of three basic single-stage .. Navigation. Main page · Contents · Featured content · Current events · Random article · Donate to Wikipedia · Wikipedia store. The common collector amplifier has quite a large current gain, larger than any other transistor Input or output phase relationship, Zero degree.
Simply calculate the value of the resistor for the current required. Emitter follower input resistance: AC coupled emitter follower, common collector circuit It is not always possible to directly couple the emitter follower, common collector buffer.
When this is the case, it is necessary to add coupling capacitors and bias resistors to the circuit. AC coupled emitter follower circuit The emitter follower can be designed using the design flow below as a basis: As before, the transistor type should be chosen according to the anticipated performance requirements. Choosing an emitter voltage of about half the supply voltage to give the most even range before the onset of any clipping, determine the current required from the impedance of the following stage.
Determine the base voltage: The base voltage is simply the emitter voltage plus the base emitter junction voltage - this is 0. Determine base resistor values: Then select the correct ratio of the resistors to provide the voltage required at the base. 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 external resistance is often ignored as this is likely to not to affect the circuit unduly. 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. 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.
The emitter follower circuit is particularly useful for applications where a input high impedance is required. Offering a high input impedance and low output impedance it is does not load circuits that may only have a small output capability, or those circuits like oscillators that need a high impedance load to ensure the optimum stability, etc. Emitter follower practical aspects When using the emitter follower circuit, there are a useful few practical points to note: If this junction is forward-biased the transistor conducting current in either its active or saturated modesit will have a voltage drop of approximately 0.
Emitter voltage follows base voltage less a 0.
In other words, the load voltage will always be about 0. Cutoff occurs at input voltages below 0. Because of this behavior, the common-collector amplifier circuit is also known as the voltage-follower or emitter-follower amplifier, because the emitter load voltages follow the input so closely. When this is done, the result is the non-inverting amplifier in Figure below.
Working and Applications of Common Collector Amplifier
The output is the same peak-to-peak amplitude as the input. Output V3 follows input V1 less a 0. Common collector non-inverting voltage gain is 1.
In the common-collector configuration, though, the load is situated in series with the emitter, and thus its current is equal to the emitter current.
- Common collector
- The Common-collector Amplifier
- Transistor Common Emitter Amplifier
With the emitter carrying collector current and base current, the load in this type of amplifier has all the current of the collector running through it plus the input current of the base. The gain calculations are all the same, as is the non-inverting of the amplified signal. The only difference is in voltage polarities and current directions shown in Figure below. PNP version of the common-collector amplifier.
A popular application of the common-collector amplifier is for regulated DC power supplies, where an unregulated varying source of DC voltage is clipped at a specified level to supply regulated steady voltage to a load.
Of course, zener diodes already provide this function of voltage regulation shown in Figure below. Zener diode voltage regulator.
However, when used in this direct fashion, the amount of current that may be supplied to the load is usually quite limited. In essence, this circuit regulates voltage across the load by keeping current through the series resistor at a high enough level to drop all the excess power source voltage across it, the zener diode drawing more or less current as necessary to keep the voltage across itself steady.
For high-current loads, a plain zener diode voltage regulator would have to shunt a heavy current through the diode to be effective at regulating load voltage in the event of large load resistance or voltage source changes. One popular way to increase the current-handling ability of a regulator circuit like this is to use a common-collector transistor to amplify current to the load, so that the zener diode circuit only has to handle the amount of current necessary to drive the base of the transistor.
Transistor Emitter Follower, Common Collector Amplifier
Figure below Common collector application: If this is the case, multiple transistors may be staged together in a popular configuration known as a Darlington pair, just an extension of the common-collector concept shown in Figure below.
An NPN darlington pair. Darlington pairs essentially place one transistor as the common-collector load for another transistor, thus multiplying their individual current gains. The overall current gain is as follows: Voltage gain is still nearly equal to 1 if the entire assembly is connected to a load in common-collector fashion, although the load voltage will be a full 1.