Common Emitter-Common Base-Common Collector Amplifiers, Physics tutorial

Common emitter amplifier:

The Common-Emitter amplifier as illustrated above provides a high gain and an inverted output. The gain may vary from one device to the other and has a strong dependence on temperature and the bias current. The gain is unpredictable and instability is a frequent problem resulting from unintentional positive feedback that may be present in the circuit. Some other problems associated with the Common Emitter circuit are the low input dynamic range imposed by the small-signal limit; there is high distortion if this limit is exceeded and the transistor ceases to behave like its small-signal model.

Negative feedback is applied through emitter degeneration. By this we mean that a small impedance is added in series with the common terminal which reduces the overall transconductance of the circuit as well as the voltage gain. This is illustrated below.

With the emitter degeneration resistor RE in series with the emitter, transconductance becomes reduced by a factor of gmRE + 1, which makes the voltage gain

871_voltage gain.jpg

So voltage gain depends approximately exclusively on ratio of resistors RC / RE rather than transistor's intrinsic and changeable characteristics. Distortion and stability characteristics of circuit are therefore improved at expense of the reduction in gain. At low frequencies small-signal characteristics are:

387_small-signal characteristics.jpg

If emitter degeneration resistor is not present, RE = 0 ohms when RE is increased, input impedance is increased and voltage gain AV is decreased.

Bandwidth of common-emitter amplifier tends to be low because of high internal capacitance. This large capacitor really decreases bandwidth of amplifier and problem can be eliminated in any one of the given ways.

  • By decreasing voltage gain through emitter degeneration
  • By decreasing signal source impedance
  • By inserting the low input impedance current buffer
  • Using an emitter follower to reduce the Miller effect is removed.

Common- emitter amplifier output is 180o out of phase with input signal. Input signal is applied across the ground and base circuit of transistor. Output signal seems across ground and collector of transistor. As emitter is connected to ground, it is common to signals, input and output. Common- emitter circuit is the most extensively utilized configuration in junction, transistor amplifiers. When compared with common-base connection, the common emitter has higher input impedance and lower output impedance. The single power supply is easily utilized for biasing circuit and in addition, higher voltage and power gains are available from common-emitter amplifiers.

Current gain in common emitter circuit is attained from base and collector circuit currents. As a very small change in base current generates large change in collector current, current gain (β) is always greater than unity for common-emitter circuit; a typical value is approx 50 and it is not odd to get values of 300 from PNP transistors. Applications of Common Emitter Amplifier range from low frequency audio amplification through radio frequency tuners.

Common base amplifier:

The common-base amplifier which is also known as grounded-base amplifier is one of high-frequency amplifiers in the Very High Frequency and Ultra High Frequency range because its input capacitance does not degrade bandwidth and because of the relatively high isolation between the input and output which translates into very little feedback from the output back to the input. This ensures high stability.

Common Base configuration finds application as current buffers since they have a current gain of approximately unity and very often, a common base amplifier is preceded by a common-emitter stage. The combination of these two forms the cascade configuration, which possesses several of the benefits of each configuration, such as high input impedance and isolation

1087_Common Base circuit characteristics.jpg

The range of allowed output voltage swing in this amplifier is tied to its voltage gain when a resistor load RC is employed for voltage amplification, Large voltage gain requires large RC, and that in turn implies a large DC voltage drop across RC .and for a given supply voltage, the larger this drop, the smaller the transistor VCB and the less output swing is allowed before saturation of the transistor occurs. Saturation results in distortion of the output signal and to avoid this situation an active load can be used as in for example, a current mirror. If this choice is made, the value of RC is replaced by the small-signal output resistance of the active load, which is generally at least as large as the output resistance of the active transistor. On the other hand, the DC voltage drop across the active load is a fixed low value which is much less than the DC voltage drop incurred for comparable gain using a resistor RC. An active load imposes less restriction on the output voltage swing.

When used as a current buffer, gain is not affected by RC, but output resistance is. Because of the current division at the output, it is desirable to have an output resistance for the buffer much larger than the load RL being driven so that large signal currents can be delivered to a load. If a resistor RC is used, a large output resistance is coupled to a large RC, again limiting the signal swing at the output. An active load provides high AC output resistance with much less serious impact upon the amplitude of output signal swing

Common collector amplifier:

The circuit can be explained by viewing transistor as being under control of negative feedback and from this point of view common-collector stage is amplifier with full series negative feedback. In this configuration with β = 1, entire output voltage VOUT is placed contrary and in series with input voltage VIN. Therefore two voltages are subtracted according to Kirchhoff's Voltage Law and their difference Vdiff = VIN - VOUT is applied to base-emitter junction.

Transistor monitors continuously Vdiff and adjusts its emitter voltage almost equal (less VBEO) to input voltage by passing according collector current through emitter resistor RE. Thus, output voltage follows input voltage variations from VBEO up to V+; that gives amplifier the name, emitter follower.

Intuitively, this behaviour can be also understood by realizing that base-emitter voltage in bipolar transistor is very insensitive to bias changes, so any change in base voltage is more or less transmitted directly to emitter. It depends slightly on different individual transistor circuit characteristics and environmental variables such as tolerances, temperature variations, load resistance, collector resistor if it is added, etc. As transistor reacts to the disturbances and restore equilibrium. It never saturates even if input voltage reaches positive supply rail. Common collector circuit can be shown mathematically to have a voltage gain of almost unity.

1858_Common collector amplifier characteristics.jpg

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