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Differential Transistor Amplifier
This page covers a differential amplifier using transistors. There are two different circuits presented; one with two differential inputs and a differential amplifier with a single input. However both circuits operate about the same, each using two transistors.
Both circuits use NPN transistors as the amplifiers; however no part number is provided, as almost any transistor would be suitable. To that end, a transistor might be selected based on its voltage range, frequency range package style or cost. A suitable part might be a 2N2484 NPN Transistor, which includes operational data on this site. The resistor and transistors are labeled so they may be noted in the descriptions, but no resistor values are given either.
Emitter Follower
The basic Emitter follower operates as a buffer, producing an output that follows the input signal but reduced by a small diode drop across the transistor. As the input signal becomes positive the transistor conducts and generates a signal across the emitter resistor. |  Emitter Follower |
The output signal follows the input as long as the input voltage is above the diode drop of the Base-Emitter junction. The emitter follower is used as a starting point because it produces an output in phase with the input and helps with the basic understanding of the differential amplifier discussed below.
Single Input Differential Amplifier
 Single Input Single Output | There is a single input applied to the Base of Q1, with a single output taken from the Collector of Q2. The transistors are operated off a dual voltage supply. The collector terminal is supplied by the positive voltage of Vcc. The Emitter terminal is supplied by the negative voltage of Vee. |
The input is applied to the Base of Q1 and developed across resistor R1. As the voltage on R1 goes more positive the current through Q1 increases. As current through the transistor increases a positive voltage is developed at the top of R3. Resistor R3 also acts as the emitter resistor for Q2. The rising voltage on the emitter of Q2 decreases the current through Q2 because the base of the transistor is grounded. The voltage drop across R4 decreases as the current through Q2 declines. As the voltage across R4 drops, the voltage at the bottom of R4 increases, which means the output voltage becomes more positive.
The reverse as describe above occurs as the input signal goes negative. As the input voltage goes negative, the voltage across R1 goes negative and the current through Q1 decreases. With the input going negative, the output of the Emitter of Q1 goes negative, which is the voltage at the top of R3. As the R3 resistor goes more negative the current through Q2 begins to increase. As the current through Q2 increases the voltage drop over R4 increases. With the increasing voltage across R4, the output, or voltage at the bottom of the resistor decreases.
Basically the operation described is the same as a single transistor emitter-follower amplifier, with the exception that this circuit provides gain and the common emitter follower does not. Note there are no blocking capacitors and the circuit uses a dual supply so the circuit will amplify both AC and DC signals, above or below zero volts.
Single Input Differential Outputs
 Differential Outputs | The next stage in the progression of the amplifier design is to explain the same circuit using the two possible outputs. The same circuit is used, but this time uses both possible outputs, or the differential outputs, Q2 & Q1. Output 1 is the new signal output, which went unused in the first circuit. As a common emitter shown to the right. |
Describing the operation in the same way as before; as the input goes positive, current through Q1 increases. The voltage drop across R2 increases and the voltage at the collector R2 node decreases. The circuit diagram shows the relative amplitude and polarity of the signal at the different points in the circuit. The two output signals have the same amplitude but opposite polarities. An additional resistor [R5] is shown connected between the two outputs at R2 and R4. The output taken across the transistors basically combines both outputs, doubling the amplitude. The new output would be twice either output and in-phase with the input signal. Next section Dual Input Differential Transistor Amplifier, using the same circuit and both inputs.
Transistor Topics
Transistor Characteristics
Electrical specifications and thermal data.
Transistor Manufacturers
List of companies making transistors and the types the makes
Transistor Package Styles
Different types of transistor packages and descriptions
Transistor Array Packages
Examples of a few transistor packages used with arrays
Transistor Definitions
Terms and acronyms used with transistors
Basic Transistor Configurations
Brief descriptions of the general transistor setups.
Transistor Audio Amplifier
Brief description of an audio amplifier using a transistor.
