Types of Coupling for Multi-Stage Transistor Amplifiers !

Types of Coupling for Multi-Stage Transistor Amplifiers !
Types of Coupling for Multi-Stage Transistor Amplifiers !
Types of Coupling for Multi-Stage Transistor Amplifiers !
Types of Coupling for Multi-Stage Transistor Amplifiers !

Various different types of stage coupling are defined depending on the components used for interconnecting the different stages of a multi-stage transistor. The components most often used for coupling are cables, resistors, capacitors, and transformers.

All the following types of coupling will be explained briefly:

  • Direct coupling
  • RC coupling (capacitive coupling if coupling only contains C and no resistors are used)
  • Impedance coupling
  • Transformer coupling 


Direct Coupling:

Direct coupling basically means interconnecting each stage directly with a cable. Typically, it is used for interconnecting common-emitter stages with other emitter-follower stages. In the following diagram showing a two-stage amplifying circuit, the output of T1 is directly connected to the input (base) of T2. The network of resistors is a voltage divider used to provide the bias and operating voltages for T1 and T2. The DC collector voltage of the first stage provides the base-bias voltage for the second stage and there are no coupling or by-pass capacitors.

Direct coupling provides good response at low frequency, since no frequency-sensitive components such as capacitors or coils are used. The frequency response of the circuit is only affected by the amplifying component itself. Increased reactance of coupling and by-pass capacitors at very low frequencies becomes excessive for practical capacitor values and produces signal loss and gain reduction.

The main disadvantage of direct coupling is the power supply requirements of the subsequent stages, because each succeeding stage requires a higher voltage. The load and voltage divider resistors use a large amount of power and the biasing (establishment of operating points) can become very complicated. Additionally, it becomes very complicated to match the impedance from stage to stage. We can say that the losses increase as the number of stages increases, so direct coupling is not actually used very often. A further disadvantage is that small changes in the DC bias voltages, due to temperature effects or power supply variation, are amplified by succeeding stages, which can result in some significant drift in the DC levels throughout the circuit.

Types of Coupling for Multi-Stage Transistor Amplifiers !

Bootstrap circuit:

The term 'bootstrap circuit' means an arrangement of components is used to increase the input impedance of an analog circuit. This is done by using a small amount of positive feedback, usually over two stages. As shown in the theory to multi-stage amplifiers, bipolar transistors inherently have a low input impedance, while many applications require very high input impedances. The need for such arrangements has largely been alleviated by using modern field effect transistors, except when extremely high input impedances are required. However, because the feedback applied is positive, such circuits usually suffer from poor stability and noise performance, compared to ones that do not bootstrap.


RC Coupling:

This is the most commonly used type of coupling. It allows for the decoupling of polarisation effects between stages. It also allows for more freedom in the design, because one stage will not affect the next one. This means capacitive coupling prevents the DC bias of one stage from affecting that of the other. In the following diagram, the circuit involving R3, R5 and C1 is the coupling network. R3 acts as a load resistor for transistors T1 and T2. For DC, the capacitor "blocks" the DC at T1's collector, but for AC, the signal can "pass". R5 outputs this "passed" or "coupled" signal as the input signal to T2 (second stage). This solves many of the problems associated with direct coupling.

RC coupling, however, does have a few disadvantages. The resistors use DC power and so the amplifier has low efficiency. The capacitor tends to limit the low-frequency response of the amplifier and the amplifying device itself limits the high-frequency response. For audio amplifiers this is usually not a problem, since frequency limitations can be overcome by using other techniques.

Types of Coupling for Multi-Stage Transistor Amplifiers !

Impedance Coupling:

This type of coupling is very similar to RC coupling, as you can observe by comparing this new diagram to the previous one. Instead of having a resistor here, the coil L1 is the load for T1 and handles the output signal of the first stage. Since the DC resistance of the coil is low, the efficiency of the amplifier stage is increased. The amplitude of the signal provided by the output of the stage depends on the inductive reactance of L1. Load inductors are most effective at high frequencies because the inductance of the coil increases with frequency. This explains why impedance coupling is not used for audio amplifiers. The other components in the network continue working in a similar way to their counterparts in the RC-coupling network (C1 and R3). C1 couples the signal between stages while blocking the DC and the input signal to the second stage is taken across R5.

Types of Coupling for Multi-Stage Transistor Amplifiers !

Transformer Coupling:

This type of coupling is very popular for amplifiers handling radio frequencies (RF). Selecting the turn ratio in the transformer appropriately allows either the voltage or the current to be increased. We will use the identifiers Q1 and Q2 for the transistors here, so as not to confuse them with the transfomers. The transformer action of T1 couples the signal from the first stage to the second stage. In the following figure, the primary of T1 acts as the load on the first stage Q1 and the secondary of T1 acts as the impedance for the second stage Q2. The transformer action couples the signal between the primary and the secondary of T1.

Types of Coupling for Multi-Stage Transistor Amplifiers !

The inductors that make up the primary and secondary of the transformer have very low DC resistance, so the efficiency of the amplifiers is very high. Transformer output is often used for the final output (between the final amplifying stage and the output device) because of the impedance-matching qualities of the transformer.

Transformer coupling is often used in high-frequency amplifiers such as those in the RF (radio frequency) and IF (intermediate frequency) sections of radio and TV receivers. For lower frequency ranges such as audio, the size of the transformers is usually prohibitive. Capacitors are usually connected across the primary windings of the transformers to obtain resonance and increased selectivity for the band of frequencies to be amplified. 

More From Iamtechnical.com