What is a BJT Differential Amplifier?

A BJT differential amplifier is a circuit that amplifies the difference between two input signals while rejecting any common-mode signals. It consists of two identical BJTs, usually NPN transistors, connected in a symmetrical configuration. The input signals are applied to the base terminals of the transistors, and the output is taken from the collector terminals.

The key characteristics of a BJT differential amplifier include:
– High input impedance
– Low output impedance
– High common-mode rejection ratio (CMRR)
– Wide bandwidth
– Good linearity

How Does a BJT Differential Amplifier Work?

The basic working principle of a BJT differential amplifier relies on the symmetrical configuration of the two transistors and the use of a constant current source. Let’s examine the circuit in more detail.

Circuit Configuration

A typical BJT differential amplifier circuit consists of the following components:
– Two matched NPN transistors (Q1 and Q2)
– Two equal-valued collector resistors (Rc1 and Rc2)
– Two equal-valued emitter resistors (Re1 and Re2)
– A constant current source (Iee)

The input signals, Vin1 and Vin2, are applied to the base terminals of Q1 and Q2, respectively. The constant current source, Iee, ensures that the total emitter current remains constant, regardless of the input signal levels.

Common-Mode Rejection

One of the key features of a differential amplifier is its ability to reject common-mode signals. A common-mode signal is a signal that appears simultaneously and equally on both input terminals. In an ideal differential amplifier, the output should be zero when both inputs receive the same signal.

In a BJT differential amplifier, the common-mode rejection is achieved through the symmetrical configuration of the transistors and the use of a constant current source. When a common-mode signal is applied to both inputs, the emitter currents of Q1 and Q2 change by the same amount. As a result, the voltage drops across the emitter resistors (Re1 and Re2) remain the same, and the output voltage remains unchanged.

Differential-Mode Amplification

When a differential signal is applied to the inputs, the BJT differential amplifier amplifies the difference between the two input signals. For example, if Vin1 is slightly higher than Vin2, the emitter current of Q1 will increase, while the emitter current of Q2 will decrease. This difference in emitter currents leads to a difference in the collector currents, which in turn creates a differential output voltage across the collector resistors (Rc1 and Rc2).

The differential-mode gain (Ad) of a BJT differential amplifier can be calculated using the following formula:

Ad = gm × Rc

where:
– gm is the transconductance of the transistors
– Rc is the value of the collector resistors

Small-Signal Analysis

To analyze the small-signal behavior of a BJT differential amplifier, we can use the hybrid-π model of the transistors. The hybrid-π model represents the transistor as a voltage-controlled current source, with an input resistance (rπ) and an output resistance (ro).

Using the hybrid-π model, we can derive the following small-signal parameters for a BJT differential amplifier:

Parameter	Formula
Input resistance (Rin)	Rin ≈ 2 × rπ
Output resistance (Rout)	Rout ≈ Rc
Differential-mode gain (Ad)	Ad ≈ gm × Rc
Common-mode gain (Acm)	Acm ≈ 0 (ideally)
Common-mode rejection ratio (CMRR)	CMRR =

Applications of BJT Differential Amplifiers

BJT differential amplifiers find applications in various analog circuits, including:

Operational Amplifiers (Op-Amps)

Op-amps are high-gain differential amplifiers with a wide range of applications, such as signal conditioning, filtering, and mathematical operations. The input stage of an op-amp typically consists of a BJT differential amplifier, which provides high input impedance and low noise.

Instrumentation Amplifiers

Instrumentation amplifiers are used to amplify small differential signals in the presence of large common-mode voltages. They consist of two or more BJT differential amplifiers connected in a cascade configuration, along with input and output buffer stages. Instrumentation amplifiers are commonly used in medical equipment, data acquisition systems, and precision measurement devices.

Comparators

Comparators are circuits that compare two input signals and produce a digital output based on their relative levels. A BJT differential amplifier can be used as the input stage of a comparator, providing high sensitivity and fast response times. Comparators are used in various applications, such as analog-to-digital converters (ADCs), level detectors, and switching power supplies.

Frequently Asked Questions (FAQ)

1. What is the purpose of the constant current source in a BJT differential amplifier?
   The constant current source ensures that the total emitter current remains constant, regardless of the input signal levels. This helps to maintain the symmetry of the circuit and improves the common-mode rejection.

2. How does a BJT differential amplifier reject common-mode signals?
   A BJT differential amplifier rejects common-mode signals through its symmetrical configuration and the use of a constant current source. When a common-mode signal is applied to both inputs, the emitter currents of the transistors change by the same amount, resulting in no change in the output voltage.

3. What is the difference between differential-mode gain and common-mode gain?
   Differential-mode gain is the amplification of the difference between the two input signals, while common-mode gain is the amplification of the common-mode signal. In an ideal differential amplifier, the differential-mode gain should be high, and the common-mode gain should be zero.

4. What is the common-mode rejection ratio (CMRR), and why is it important?
   The common-mode rejection ratio (CMRR) is the ratio of the differential-mode gain to the common-mode gain. A high CMRR indicates that the differential amplifier effectively rejects common-mode signals while amplifying the differential signal. This is important in applications where small differential signals need to be measured in the presence of large common-mode voltages.

5. What are some common applications of BJT differential amplifiers?
   BJT differential amplifiers are used in various analog circuits, including operational amplifiers (op-amps), instrumentation amplifiers, and comparators. They are essential building blocks in signal conditioning, precision measurement, and data acquisition systems.

Conclusion

BJT differential amplifiers are essential building blocks in analog electronics, providing high input impedance, low output impedance, and excellent common-mode rejection. By understanding the working principles and characteristics of BJT differential amplifiers, engineers and designers can effectively utilize them in various applications, such as op-amps, instrumentation amplifiers, and comparators.

As technology advances, BJT differential amplifiers continue to play a crucial role in the development of high-performance analog circuits. With their ability to amplify small differential signals while rejecting common-mode noise, these versatile amplifiers enable the design of precise and reliable electronic systems across a wide range of industries.