Understanding the Bandwidth Range of a Common Collector BJT Amplifier

Introduction to the Common Collector BJT Amplifier

A common collector Bipolar Junction Transistor (BJT) amplifier, also known as an emitter follower, is a popular circuit topology due to its high input impedance, low output impedance, and unity voltage gain. Understanding the bandwidth range of this amplifier is crucial for its successful implementation in various applications, such as audio and RF systems.

Defining the Bandwidth of a Common Collector BJT Amplifier

The bandwidth (BW) of a common collector amplifier is defined as the frequency range over which the amplifier can operate effectively. It is typically measured from the lower cutoff frequency (f_L) to the upper cutoff frequency (f_H). The amplifier's performance degrades outside this frequency range due to the introduction of attenuation and phase shifts.

Typical Bandwidth Range of a Common Collector Amplifier

The bandwidth of a common collector amplifier can vary significantly, ranging from a few hertz to several megahertz. The exact bandwidth depends on the specific circuit design, the characteristics of the transistor used, and the load impedance:

Low-Frequency Applications: The lower bandwidth (f_L) might be as low as a few hertz, suitable for very low-frequency or DC applications. Audio and RF Applications: The upper bandwidth (f_H) can extend up to several megahertz, making it suitable for audio and RF applications where high-frequency performance is required.

A typical practical bandwidth for a common collector amplifier might be from about 10 Hz to 1 MHz.

Factors Influencing the Bandwidth of a Common Collector Amplifier

Transistor Parameters

Transition Frequency (f_T): The transition frequency is a critical parameter that limits the upper bandwidth of the amplifier. It is the frequency at which the transistor starts to provide inadequate gain for the signal.

Current Gain (h_FE): The current gain of the transistor also affects the bandwidth. Higher gain transistors are more likely to have higher bandwidth.

Load Impedance

The load impedance connected to the emitter of the common collector amplifier significantly affects the lower and upper cutoff frequencies. A lower load impedance can reduce the bandwidth, leading to a narrower usable frequency range.

Capacitive Coupling and Bypass Capacitors

Coupling and bypass capacitors can introduce high-pass and low-pass filtering effects, shaping the overall bandwidth. For example, a coupling capacitor can act as a low-pass filter, attenuating high-frequency signals, while a bypass capacitor can provide a low impedance path at high frequencies, reducing phase shifts.

Parasitic Capacitances

The internal capacitances of the transistor, such as the base-collector capacitance, can limit the upper frequency response, further reducing the bandwidth.

Calculating the Bandwidth

To determine the specific bandwidth of a common collector amplifier, you can analyze the frequency response using Bode plots or by calculating the -3 dB points based on the circuit's transfer function. The -3 dB point defines the frequency at which the gain drops to half its maximum value (3 dB loss).

Example Application and Measurement

The bandwidth of a common collector amplifier depends on the transistor used. RF transistors are available in the GHz range and can provide superior performance at high frequencies. However, for general-purpose transistors like BC107, the bandwidth is typically around 100 MHz.

In a typical university laboratory, the function generator's maximum range might be only up to 10 MHz. Therefore, you may not be able to measure the full bandwidth characteristics. Instead, you can use PSPICE simulations to predict the behavior of the amplifier over its bandwidth range.

It is essential to consider the unity voltage gain of the common collector amplifier. By utilizing the full bandwidth of the transistor, you can achieve the highest performance possible, as the gain-bandwidth product remains constant.