BJT: Differential Amplifier Explained

I. Wideband Differential Amplifier Calculator

This tool is designed to calculate critical performance metrics for wideband differential amplifiers, including current levels, bias voltage, gain, and frequency response.

What is a Wideband Amplifier?

A Wideband Differential Amplifier is an amplifier designed to maintain a precise amplification factor over a wide frequency range while providing high gain.

By using a specific configuration—tying the collector of the first transistor ($Q_1$) to the base of the second ($Q_2$) to signal ground—it is possible to overcome the Miller Capacitance limitations. The resulting gain-bandwidth product is orders of magnitude greater than that of standard common-emitter amplifiers.

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II. Understanding Differential Amplifiers

2.1 What is a Differential Amplifier?

A Differential Amplifier (or difference amplifier) is a circuit that amplifies the difference between two input signals while rejecting any signals that are common to both inputs. It is the fundamental building block of the Operational Amplifier (Op-Amp).

The Formula: The basic output equation is:

$V_{out} = A_d \times (V_1 - V_2)$

Where:

• $V_{out}$: Output Voltage
• $A_d$: Differential Gain
• $V_1, V_2$: Input Voltages

Ideally, if $V_1 = V_2$, the output is zero. The amplifier focuses purely on the gap between the two voltages.

2.2 Advantages

• Drift Elimination: They are excellent at minimizing "drift" (slow changes in voltage due to temperature fluctuations), making them ideal for precision instruments like oscilloscopes and voltmeters.
• Noise Rejection: They respond only to the difference signal and ignore Common-Mode Signals (noise or ground voltages present on both inputs).

2.3 Why are they used? (Noise Suppression)

Differential amplifiers are primarily used to suppress Noise.

• Common-Mode Noise: Electrical noise (from electromagnetic induction or ground loops) often affects both input wires equally.
• The Solution: Since the amplifier subtracts one input from the other ($V_1 - V_2$), any noise present on both lines is effectively cancelled out. This is vital in electrically noisy environments where weak signals can easily be corrupted.

2.4 Applications

• Medical Electronics: Amplifying weak bio-signals (like heart signals/ECG) for digital processing.
• Audio Systems: Used in volume control circuits and balanced audio lines.
• Modulation: Essential for amplitude modulation techniques.
• Automatic Gain Control (AGC): Used in control circuits requiring stable gain.

2.5 Configuration Types

1. Dual Input, Balanced Output: Two inputs, differential output.
2. Dual Input, Unbalanced Output: Two inputs, single-ended output.
3. Single Input, Balanced Output: One input, differential output.
4. Single Input, Unbalanced Output: One input, single-ended output.

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III. Differential Amplifier Circuit Implementations

3.1 MOSFET Differential Amplifier

• Structure: Uses Metal-Oxide-Semiconductor Field-Effect Transistors (MOSFETs).
• Operation: The terminals (Sources) of both MOSFETs are connected to a constant current source.
• Behavior: If the input voltage exceeds the threshold, the MOSFET enters the active region (triode/saturation depending on load). The circuit relies on the balance between the two MOSFETs to steer current.

3.2 BJT Differential Amplifier

• Structure: Uses Bipolar Junction Transistors (BJTs). The Emitters of both transistors are tied together and connected to a common current source or ground resistor.
• Operation: Inputs are applied to the Bases; the output is typically taken from the Collectors.
• Mechanism: When an input is applied to Base 1 ($Q_1$), the voltage drop across the emitter resistor changes, altering the conduction of $Q_1$. Because the emitters are coupled, this change affects $Q_2$, allowing the circuit to amplify the difference signal.

3.3 Op-Amp Differential Amplifier

• Structure: Built using a standard Operational Amplifier with external resistors.
• Operation: Uses both the inverting (-) and non-inverting (+) terminals.
• Gain Control: Unlike raw transistor circuits, the gain here is easily determined by the ratio of external resistors.
• Note: For perfect differential operation, the ratio of the resistors on the inverting side must match the ratio on the non-inverting side.