What is a BJT Differential Amplifier?
A differential amplifier, as the name suggests, amplifies the difference between two input voltages. It has two inputs and one output. The output voltage is ideally proportional to the difference between the two input voltages:
Vout = Ad * (Vin+ – Vin-)
where Ad is the differential gain of the amplifier.
A BJT differential amplifier uses bipolar junction transistors (BJTs) to implement the amplification. BJTs are current-controlled devices – a small current flowing into the base terminal controls a much larger collector current. By connecting BJTs in a differential pair configuration, we can harness this amplification to create a high-gain differential amplifier.
How a BJT Differential Amplifier Works
The core of a BJT differential amplifier is the differential pair – two matched transistors with their emitters connected together. Here’s the basic topology:
Q1 and Q2 are the matched transistors forming the differential pair. Their emitters are joined together and connected to a constant current source I_EE. The two input voltages Vin+ and Vin- are applied to the bases of Q1 and Q2 respectively. The collectors of Q1 and Q2 are connected to load resistors Rc1 and Rc2, and the output is taken from one of the collectors.
Here’s how it works:
- The constant current I_EE is split between the two transistors Q1 and Q2 based on the input voltages Vin+ and Vin-.
- If Vin+ is higher than Vin-, more of I_EE flows through Q1 than Q2. This makes the collector current of Q1 (Ic1) increase and the collector current of Q2 (Ic2) decrease.
- The changes in collector currents cause the collector voltages to change in the opposite direction, because of the voltage drops across the load resistors. Vc1 decreases and Vc2 increases.
- The difference between the collector voltages (Vc1 – Vc2) is therefore proportional to the input differential voltage (Vin+ – Vin-). This is the amplified output of the differential amplifier.
If the two input voltages are equal (Vin+ = Vin-), then the currents through Q1 and Q2 are equal and the collector voltages are equal. The output in this case is zero. Any common-mode signal that appears on both inputs gets canceled out in the output – this is common-mode rejection, a key feature of differential amplifiers.
Key Characteristics and Parameters
Several key parameters characterize the performance of a BJT differential amplifier:
Differential Gain (Ad)
This is the ratio of the change in output voltage to the change in differential input voltage. For the basic differential pair,
Ad = gm * Rc
where gm is the transconductance of the transistors and Rc is the collector load resistance. The differential gain can be quite high, on the order of 100s or 1000s.
Common-Mode Rejection Ratio (CMRR)
CMRR indicates how well the amplifier rejects common-mode signals. It’s defined as the ratio of differential gain to common-mode gain:
CMRR = |Ad / Acm|
where Acm is the common-mode gain, i.e., the ratio of output voltage change to common input voltage change. Ideally, Acm should be zero and CMRR should be infinite. In practice, CMRR values of 80-100 dB are achievable with careful design.
Input Impedance
The input impedance of a BJT differential amplifier is relatively low, as the input signal has to drive current into the base-emitter junctions of the transistors. The input impedance for each input is approximately:
Zin ≈ β * (re + 1/gm)
where β is the current gain of the transistor, re is its intrinsic emitter resistance, and gm is its transconductance. The low input impedance can be a limitation in some applications.
Output Impedance
The output impedance is the impedance seen looking back into the output of the amplifier. For the basic differential pair, it equals the collector load resistance Rc. Using active loads instead of resistors can increase the output impedance.
Frequency Response
BJT differential amplifiers have limited frequency response due to several factors:
- The internal capacitances of the transistors (Cbe, Cbc) shunt the input and output at high frequencies
- The load resistors and stray capacitances create low-pass filtering
- Mismatches between the transistors cause the CMRR to deteriorate at high frequencies
The −3dB bandwidth of a BJT differential amplifier is typically in the range of MHz to 10s of MHz. Careful design and compensation techniques can extend the bandwidth.
Modifications and Enhancements
Several techniques can be used to enhance the performance of BJT differential amplifiers:
Active Loads
Replacing the collector resistors Rc1 and Rc2 with active loads (current sources) increases the differential gain and output impedance of the amplifier. This is because the effective load resistance seen by the differential pair becomes very high.
Emitter Degeneration
Adding resistors Re in series with the emitters of Q1 and Q2 introduces negative feedback that stabilizes the bias point and increases the input impedance. It also linearizes the transfer characteristic of the amplifier. However, it reduces the differential gain.
Darlington Configuration
Connecting another transistor in cascade with each transistor of the differential pair (Darlington configuration) increases the input impedance and current gain of the amplifier. This is useful when the input signal is weak or from a high-impedance source.
Cascode Configuration
Adding common-base transistors on top of the differential pair (cascode configuration) increases the bandwidth and reduces the effect of transistor mismatches. It also increases the output impedance and reduces the interaction between the input and output.
Common Applications
BJT differential amplifiers find widespread use in analog circuit applications such as:
Instrumentation Amplifiers
Instrumentation amplifiers are used to amplify small differential signals in the presence of large common-mode noise, such as in sensor interfaces and biomedical equipment. They typically use two or three stages of BJT differential amplifiers to achieve high gain, high CMRR, and high input impedance.
Operational Amplifiers
Most operational amplifiers (op-amps) use BJT differential amplifiers as the input stage. The differential amplifier provides the high gain and common-mode rejection needed for the op-amp to function as a versatile analog building block.
Audio Amplifiers
BJT differential amplifiers are used in the input stages of many audio amplifiers to convert the audio signal from single-ended to differential form. This helps to reject noise and interference picked up by the audio cables.
Data Converters
Analog-to-digital converters (ADCs) and digital-to-analog converters (DACs) often use BJT differential amplifiers in their front-end circuitry for signal conditioning and buffering. The low noise and high linearity of BJT differential amplifiers help to preserve the signal integrity.
Frequently Asked Questions (FAQ)
1. What is the difference between a BJT differential amplifier and an op-amp?
A BJT differential amplifier is a single-stage amplifier that amplifies the difference between two input voltages. An op-amp, on the other hand, is a multi-stage amplifier that uses a BJT differential amplifier as its input stage, followed by additional gain stages and an output stage. Op-amps have much higher gain, bandwidth, and input impedance compared to a single BJT differential amplifier.
2. Can a BJT differential amplifier be used with a single-ended input?
Yes, a BJT differential amplifier can be used with a single-ended input by grounding one of the inputs (usually the inverting input). In this case, the amplifier acts as a single-ended to differential converter, rejecting any ground noise or interference. However, the common-mode rejection property is not utilized in this configuration.
3. What limits the CMRR of a BJT differential amplifier?
The CMRR of a BJT differential amplifier is limited by mismatches between the two transistors of the differential pair. These mismatches can be in terms of current gain (β), base-emitter voltage (Vbe), or collector-emitter saturation voltage (Vce,sat). Careful transistor matching and layout techniques can help to minimize these mismatches and improve CMRR.
4. How can the input impedance of a BJT differential amplifier be increased?
The input impedance of a BJT differential amplifier can be increased by using emitter degeneration (adding resistors in series with the emitters), using Darlington transistors, or using an input buffer stage. However, these techniques also have trade-offs in terms of gain, noise, and complexity.
5. What is the advantage of using active loads in a BJT differential amplifier?
Using active loads (current sources) instead of resistors as the collector loads of a BJT differential amplifier has several advantages:
- It increases the differential gain and output impedance of the amplifier, as the effective load resistance becomes very high.
- It improves the CMRR and power supply rejection ratio (PSRR) by providing better balance and isolation between the two sides of the differential pair.
- It allows for higher voltage swings at the output, as the current sources can be designed to have a high output impedance and low voltage drop.
However, active loads also increase the complexity and power consumption of the amplifier, and may require additional biasing circuitry.
Conclusion
BJT differential amplifiers are a fundamental building block in analog circuit design. They provide high gain, high common-mode rejection, and the ability to interface with differential signals. By understanding the basics of how BJT differential amplifiers work and their key characteristics, designers can effectively use them in a wide range of applications.
While the basic BJT differential pair has some limitations in terms of input impedance, output impedance, and frequency response, various modifications and enhancements can be used to improve its performance. These include using active loads, emitter degeneration, Darlington configuration, and cascode configuration.
BJT differential amplifiers find widespread use in instrumentation amplifiers, operational amplifiers, audio amplifiers, and data converters. As the demand for low-noise, high-performance analog circuits continues to grow, BJT differential amplifiers will remain an essential tool in the designer’s toolkit.
Key Takeaways
- BJT differential amplifiers amplify the difference between two input voltages while rejecting common-mode signals.
- The basic BJT differential pair consists of two matched transistors with their emitters connected together and a constant current source.
- Key parameters of BJT differential amplifiers include differential gain, common-mode rejection ratio (CMRR), input impedance, output impedance, and frequency response.
- Various modifications such as active loads, emitter degeneration, Darlington configuration, and cascode configuration can be used to enhance the performance of BJT differential amplifiers.
- BJT differential amplifiers are widely used in instrumentation amplifiers, operational amplifiers, audio amplifiers, and data converters.
By mastering the concepts and techniques of BJT differential amplifiers, analog designers can create high-performance, noise-resistant circuits for a wide range of applications in instrumentation, communication, and signal processing.
