Understanding the Differential Amplifier : Its Concept and Analysis

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In electronics technology, in addition to Single-Ended (S/E) Amplifier topologies like Common Emitter, Common Collector, and Common Base, there is a very popular topology known as the Differential Amplifier.

While a single-ended amplifier has only one input, a differential amplifier has two inputs. As its name suggests, instead of amplifying just one input, a differential amplifier amplifies the difference between its two inputs.

Both single-ended and differential amplifiers can have more than one output, either in the form of voltage or current. The choice depends on the design requirements. Thus, both can function as a voltage-to-voltage amplifier or a voltage-to-current amplifier. A voltage-to-current amplifier is also known as a transconductance amplifier.

Figure 1 Differential Amplifier

Basic Differential Amplifier Topology

Let's focus on the differential amplifier. This topology is essentially an integration of two single-ended amplifiers, each with its own separate input and load. However, a unique characteristic of a differential amplifier is that both single-ended amplifiers share a single Constant Current Source (CCS), where the current from the CCS is split between the two integrated amplifiers.

Figure 1 shows the basic topology of a differential amplifier.

  • along with and forms the left-side single-ended amplifier.

  • along with and forms the right-side single-ended amplifier.

  • The emitters of both amplifiers are tied together and supplied with current from the CCS.

The current source in the figure is simplified as a model. In a real-world implementation, it can be a simple resistor or a more complex current source circuit using active components.

When designing a differential amplifier, it's crucial to ensure that the base-emitter junction is always in a forward-biased condition. The constant current value and the impedance of the CCS must be chosen accordingly. In practice, the base is often connected to ground via a resistor to provide sufficient base current. However, the base can also be biased with a different voltage, as long as the base-emitter junction remains in a forward-biased state.

Due to this distinctive construction, a basic differential amplifier is often referred to as a Long Tail Pair (LTP). The name refers to the "long tail" of the constant current source connected to the emitters.


Principle of Operation and Current Balance

Because of this construction, when one single-ended amplifier draws more current, the current flowing through its counterpart decreases. The interesting part is that, overall, the sum of the currents flowing through the two amplifiers (through R3 and R4) always remains constant, equal to the current supplied by the Constant Current Source (CCS).

Figure 2. Currents in the Differential Amplifier with Different Input Voltages

Figure 2 shows the operational effect of a differential amplifier test rig. With a sinusoidal input on VI1 and a pulse input on VI2 the output currents IR3 and IR4 are generated. Note that the sum of these two load currents, IR3 + IR4 is always constant, at approximately 1 mA, corresponding to the current provided by the CCS.

Figure 3. NPN and PNP Differential Amplifier Basic Circuit


Additional Notes:

  • The LTP can be implemented using various types of transistors, such as PNP, NPN, JFET N, JFET P, MOSFET N, MOSFET P, and of course, vacuum tubes.

  • Figure 3 shows an LTP with PNP vs. NPN transistors.

  • A differential amplifier can be powered by a dual supply voltage such as VPOS and VNEG, or a single supply such as VPOS and GND as well as GND and VNEG. The key is that the base-emitter junction must always be forward-biased.

  • In non-critical applications, the constant current source can be replaced by a simple resistor. However, for applications requiring high precision (including immunity to noise and thermal stability), the CCS must be implemented with a more complex active current source circuit.

  • Although the example above uses a single transistor for the S/E amplifier, in practice, more complex constructions are often used to achieve optimal design characteristics such as input impedance, gain factor, zero offset voltage, and thermal stability. These might include Darlington or Sziklai pairs, or a Wilson Current Mirror as the active load. You can see these details in the schematics of operational amplifiers from various manufacturers.

I hope this article is helpful. TABIK !


(This post is parallel to the status on the FaceBookGroup The Art of Electronics with the same Topic)

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