In bipolar junction transistor, the current gain is maximum in common collector configuration but still we use the common emitter configuration (at least it is the thing that was taught to me). What's the reason behind this?
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- \$\begingroup\$ It is a widespread misconception that a high current gain would also enable a high voltage gain. This is wrong - the BJT is a voltage-controlled device. The advantage of a high current gain is that - with the same collector current - the lower base current means a higher input resistance, \$\endgroup\$LvW– LvW2024-10-28 14:03:07 +00:00Commented Oct 28, 2024 at 14:03
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2 Answers
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In a common-emitter amplifier, the BJT has a current gain that is within 1 or 2% of the current gain for a common-collector configuration (in many, many applications). Hence, it's not much different.
However, the common-collector circuit has a voltage gain that cannot be greater than unity i.e. there is no voltage amplification possible in CC hence, it has a limited use in signal amplifiers.
Common-emitter (on the other hand) can provide significant voltage gain and is very useful in the amplification of small signal-level voltages. For this reason it is very commonly used.
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Common collector (emitter follower) configuration IS used extensively. Don't believe otherwise. It is used in the form of push-pull amplifiers, in the output sections of op-amps, audio amplifiers, and various drivers, where the output impedance needs to be low. Common collector amplifiers have much lower output impedance than common emitter amplifiers.
A push pull amplifier (with cross-over distortion), might be as simple as this circuit:
simulate this circuit – Schematic created using CircuitLab
Such a circuit can be found, for example, driving the power transistors in motor H-bridge drives. Because this simple circuit has significant cross-over distortion alternative push-pull configurations are to be found in audio circuits and linear amplifiers. The next step up is to add to diodes to the input side of the push-pull, like so:
While this circuit has significantly less distortion, it suffers from wasted current flowing directly the transistors. This current furthermore increases with the temperature of the transistors. To combat this, even more circuitry is often added, such as in the following configuration.
In this circuit, Q3, R5, R6, and R7 form what is known as a "rubber diode", which provides an adjustable voltage drop between the bases of Q1 and Q2. R3 and R4 serve to stabilize the voltages between the bases of Q1 and Q2 and Vout from thermal effects. This circuit allows a sweet spot to be found in the trade-off between cross-over distortion, and wasted power.
answered Oct 28, 2024 at 13:09
Math Keeps Me Busy
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