The input impedance of a transistor connected in _________ arrangement is the highest.
common collector
Let's analyze the different transistor arrangements and their typical input impedance characteristics to determine which one has the highest input impedance.
Understanding Transistor Arrangements and Input Impedance
Transistors, like the BJT (Bipolar Junction Transistor), can be configured in three main ways for use in amplifier circuits:
- Common Emitter (CE)
- Common Base (CB)
- Common Collector (CC), also known as Emitter Follower
The input impedance of a circuit stage tells us how much the circuit opposes current flow into its input. A high input impedance means the circuit draws very little current from the source driving it, which is often desirable for minimizing loading effects.
Input Impedance of Common Transistor Configurations
Each transistor configuration has distinct input and output impedance characteristics:
- Common Emitter (CE): This configuration offers moderate input impedance and moderate output impedance. It provides voltage gain and current gain, with a 180-degree phase shift between input and output voltage. Typical input impedance is in the range of a few hundred ohms to a few kilohms.
- Common Base (CB): The common base configuration has a very low input impedance and a high output impedance. It provides voltage gain but no current gain. There is no phase shift between input and output voltage. Typical input impedance is in the range of tens to a few hundred ohms.
- Common Collector (CC): The common collector configuration is primarily used as a buffer or impedance matching circuit. It provides high current gain but the voltage gain is slightly less than 1. It has a high input impedance and a low output impedance. There is no phase shift between input and output voltage.
Why Common Collector has High Input Impedance
The high input impedance of the common collector arrangement comes from the feedback effect of the emitter resistance. The input signal is applied to the base, and the output is taken from the emitter. The voltage at the emitter follows the base voltage (minus the base-emitter voltage drop, $V_{BE}$). The current drawn from the input source is approximately the output current divided by the transistor's current gain ($\beta$ or $h_{fe}$).
If there is a load resistor ($R_L$) connected to the emitter (or the emitter itself is connected to ground via a resistor $R_E$), this resistance appears amplified at the input terminal. The effective input impedance is approximately given by:
\( Z_{in(CC)} \approx \beta \times R_E \) (if $R_E$ is the load) or \( Z_{in(CC)} \approx \beta \times R_L \) (if $R_L$ is connected to the emitter)
More accurately, including the transistor's internal emitter resistance ($r'_e$), the input impedance looking into the base is approximately \( Z_{in(base)} \approx \beta(r'_e + R_E || R_L) \). When considering the input from the source, series resistances are added.
However, the key point is that the impedance connected to the emitter is multiplied by $\beta$ (which can be 100 or more) when viewed from the base, resulting in a significantly higher input impedance compared to the other configurations.
Comparing Input Impedances
| Configuration | Input Impedance ($Z_{in}$) | Output Impedance ($Z_{out}$) | Voltage Gain ($A_v$) | Current Gain ($A_i$) |
|---|---|---|---|---|
| Common Emitter (CE) | Moderate (k$\Omega$) | Moderate (k$\Omega$) | High | High |
| Common Base (CB) | Low ($\Omega$) | High (k$\Omega$) | High | Low ($\approx$ 1) |
| Common Collector (CC) | High (k$\Omega$ to M$\Omega$) | Low ($\Omega$) | $\approx$ 1 | High |
As shown in the table and explained by the impedance reflection principle, the common collector configuration provides the highest input impedance among the three basic transistor arrangements.
Important Questions from Configuration of BJT
For the CE-transistor amplifier, the audio signal voltage across the collected resistance of 3 kΩ is 3V. Assume the current amplification factor of the transistor is 50, and find the input voltage and base current, if the resistance is 1 k Ω ?
Which of the following is NOT true for a common collector transistor?
The voltage gain of a Common emitter amplifier ______, as the load resistance is increased
Find the value of β for a BJT having α = 0.99.
The current gain of amplifier stage is lowest in
