The emitter follower, also known as a common collector amplifier, is a fundamental building block in analog electronics. It is characterized by its high input impedance, low output impedance, and unity voltage gain. This unique combination makes the emitter follower a versatile circuit suitable for a wide range of applications, from impedance matching to signal buffering. The emitter follower is particularly valuable for applications where a strong signal source needs to drive a low-impedance load, ensuring maximum power transfer and signal integrity.
Understanding the Emitter Follower Circuit
At its core, the emitter follower is a bipolar junction transistor (BJT) configured in a common collector configuration. This means the collector is connected to the supply voltage (Vcc), the base serves as the input, and the emitter is the output. A resistor (Re) is typically connected between the emitter and ground to provide a current path for the emitter current (Ie).
Key Characteristics of an Emitter Follower:
- High Input Impedance: The emitter follower exhibits a high input impedance, typically in the range of several kilohms to several megohms. This is due to the base current being amplified by the transistor's current gain (β).
- Low Output Impedance: The emitter follower has a low output impedance, typically in the range of tens to hundreds of ohms. This is because the output is taken from the emitter, which is effectively a voltage follower.
- Unity Voltage Gain: The emitter follower has a voltage gain of approximately unity, meaning the output voltage closely follows the input voltage. However, the output voltage will be slightly lower than the input voltage due to the base-emitter voltage drop (Vbe), which is typically around 0.7 volts for silicon transistors.
Operation of an Emitter Follower
The operation of an emitter follower can be explained as follows:
- Input Signal: An input signal is applied to the base of the transistor.
- Base Current: The input signal causes a base current (Ib) to flow into the base of the transistor.
- Collector Current: The base current is amplified by the transistor's current gain (β), resulting in a much larger collector current (Ic) flowing through the collector and emitter.
- Emitter Current: The emitter current (Ie) is equal to the sum of the base current (Ib) and the collector current (Ic), which is effectively the amplified base current.
- Output Voltage: The emitter voltage (Ve) is determined by the emitter current (Ie) and the emitter resistor (Re). Since the emitter current is amplified by the transistor's current gain, the output voltage closely follows the input voltage.
Applications of Emitter Followers
The emitter follower's unique characteristics make it well-suited for various applications, including:
1. Impedance Matching: The emitter follower's high input impedance and low output impedance are ideal for matching the impedance of different circuits. For example, a high-impedance signal source can be connected to the emitter follower, while the low-impedance output can drive a low-impedance load without signal attenuation.
2. Signal Buffering: The emitter follower can act as a buffer, isolating the input signal from the output load. This prevents the load from affecting the signal source, ensuring that the signal remains unchanged.
3. Level Shifting: The emitter follower can shift the voltage level of a signal. By choosing the appropriate emitter resistor value, the output voltage can be shifted up or down relative to the input voltage.
4. Current Amplification: Although the emitter follower has a voltage gain of unity, it can amplify current. The emitter current is amplified by the transistor's current gain (β).
5. Voltage Regulation: The emitter follower can be used as a simple voltage regulator. By connecting a Zener diode in the base circuit, the output voltage can be regulated to the Zener diode's voltage.
Advantages and Disadvantages of Emitter Followers
Advantages:
- High input impedance
- Low output impedance
- Unity voltage gain
- Good linearity
- Simple to implement
Disadvantages:
- Limited voltage gain
- Voltage drop across the base-emitter junction
- Can be affected by temperature variations
Conclusion
The emitter follower is a versatile and widely used circuit in analog electronics. Its high input impedance, low output impedance, and unity voltage gain make it suitable for a variety of applications, including impedance matching, signal buffering, level shifting, and current amplification. While it has limitations in terms of voltage gain and temperature sensitivity, its simplicity and effectiveness make it a valuable tool in circuit design.