When is an emitter-follower used in a BJT circuit?

When is an emitter-follower used in a BJT circuit?

In a BJT configuration when the emitter terminal is used as the output, the network is called an emitter-follower. In this configuration the output voltage is always a shade lower than the input base signal due to the inherent base to emitter drop. In simple terms, in this type of transistor circuit the emitter seems to be following

What makes an emitter follower a common collector?

The emitter follower or common collector circuit configuration provides a high input impedance and a low output impedance. This means that the emitter follower circuit provides an ideal buffer stage,…

What kind of transistor has an emitter follower?

An emitter follower is one of the standard transistor configurations which is also referred to as common collector transistor configuration. Let’s try to first understand what’s an emitter follower transisto r and why it’s called a common collector transistor circuit.

Why is a CC configuration called an emitter follower?

Emitter Follower is a configuration with the collector serving as the common terminal in both forward bias and reverse bias circuits. The CC configuration is called the Emitter Follower because the voltage gain of this configuration is 1. Hence, the output voltage is same as input voltage.

What are the input and output impedances of BJT?

In AoE’s chapter on BJT (specifically section 2.2.3B Input and Output Impedances of Emitter followers), it derives the input and output impedances to be:

How to calculate the output impedance of an emitter follower?

According to formula 2.3, the input resistance of the emitter follower looking into the base would be ( (100+1)*1k)+2M = 2,101,000ohm. With a voltage drop of 555.905mV in V _be inside the BJT, the Thevenin voltage into the base is 10V – 555.905V = 9.444095V.

Why does a BJT circuit behave like an amplifier?

A common base BJT circuit behaves like a non-inverting amplifier circuit, due to the fact that its output signals is always in-phase with the input signal, meaning the output precisely replicates the input signal’s polarity without changing its attributes.

What can be done to improve the emitter follower?

An alternate approach to improving the emitter follower is to reduce the effective r e through negative feedback. Reducing r e can be addressed by adding a second transistor to increase the negative feedback factor by increasing the open-loop-gain.

Why is the output VO called an emitter follower?

As understood earlier, the output Vo appears to be “following” the input signals levels Vi, through an in-phase relationship, and this represents its name emitter follower. The emitter-follower configuration is mainly used for impedance-matching applications, due to its high impedance characteristics at the input and a low impedance at the output.

Why do we use an emitter follower transistor?

We first use a Zener diode and resistor across the input rail to make a PD that provides a regulated output. This output from the PD then drives the base junction of the transistor so its output is regulated as well. An advantage of using an emitter follower transistor is that it allows for greater power handling, than a Zener diode could alone.

How does the Zener diode regulate the BJT?

So a low power rated zener diode can regulate the base voltage of the BJT that can pass a huge current through it. In some cases, the zener diode connected to the base of transistor will not provide enough base current. To solve this problem, an additional transistor is used as shown in the following circuit.