Table of Contents
Multistage Amplifier, RC Coupling, Transformer Coupling, Direct Coupling
In Actuality, we require an Amplifier that can boost a signal from a very weak source, like a Microphone, to a level appropriate for another transducer, like a Loudspeaker, to operate at. This is Accomplished using Multistage Amplifier, which cascade multiple Amplifier stages.
1. Need for Cascading
A faithful Amplifier should match its input and output Impedances to the source and the load, and it should have the desired voltage and current gains. Because of the Limitations of Transistor/FET Parameters, these Fundamental Requirements of the Amplifier are Frequently not met by single stage Amplifiers. In these circumstances, multiple Amplifier stages are Cascaded in order to meet Impedance Matching Requirements with some Amplification from the input and output stages while the Majority of the Amplification is provided by the remaining middle stages.
We can say that,
When the Amplification of a single stage Amplifier is not Sufficient, or,
When the input or output Impedance is not of the correct Magnitude, for a particular application two or more Amplifier stages are connected, in cascade. Such Amplifier, with two or more stages is also known as Multistage Amplifier.
Two Stage Cascaded Amplifier
Vi1 is the input of the first stage and Vo2 is the output of second stage.
So, Vo2/Vi1 is the overall voltage gain of two stage amplifier.
n-Stage Cascaded Amplifier
Voltage gain :
The resultant voltage gain of the multistage amplifier is the product of voltage gains of the various stages.
Av = Avl Av2 … Avn
Gain in Decibels
When comparing two powers, it is frequently found to be much more convenient to do so on a logarithmic scale rather than a linear one. Decibel is the name of the scale’s logarithmic measurement unit (abbreviated dB). The difference between a power P2 and a power P1 in decibels, N, is determined by
dB, or decibel, stands for power ratio. When the number of dB is negative, the power P2 is smaller than the reference power P1, and when it is positive, the power P2 is larger than the reference power P1.
For an amplifier, P1 may represent input power, and P2 may represent output power.
Both can be given as
Where Ri and Ro are the input and output impedances of the amplifier respectively. Then,
If the input and output impedances of the amplifier are equal i.e. Ri = Ro= R, then
Gain of Multistage Amplifier in dB
The gain of a multistage amplifier can be easily calculated if the gain of the individual stages are known in dB, as shown below
20 log10 Av = 20 log10 Avl + 20 log10Av2 +… + 20 log10 Avn
Thus, the overall voltage gain in dB of a multistage amplifier is the decibel voltage gains of the individual stages. It can be given as
AvdB = AvldB + Av2dB + … + AvndB
2. Advantages of Representation of Gain in Decibels
Logarithmic scale is preferred over linear scale to represent voltage and power gains because of the following reasons :
- In multistage amplifiers, it permits to add individual gains of the stages to calculate overall gain.
- It allows us to denote, both very small as well as very large quantities of linear, scale by considerably small figures. For example, voltage gain of 0.0000001 can be represented as -140 dB and voltage gain of 1,00,000 can be represented as 100 dB.
- Many times output of the amplifier is fed to loudspeakers to produce sound which is received by the human ear. It is important to note that the ear responds to the sound intensities on a proportional or logarithmic scale rather than linear scale. Thus use of dB unit is more appropriate for representation of amplifier gains.
Methods of coupling Multistage Amplifier
In multistage amplifier, the output signal of preceding stage is to be coupled to the input circuit of succeeding stage. For this interstage coupling, different types of coupling elements can be employed. These are :
1. RC coupling
2. Transformer coupling
3. Direct coupling
Figure shows RC coupled amplifier using transistors. The output signal of first stage is coupled to the input of the next stage through coupling capacitor and resistive load at the output terminal of first stage
Since the coupling capacitor Cc prevents the d.c. voltage of the first stage from reaching the base of the second stage, it has no effect on the quiescent point of the subsequent stage. It is a broadband network, the RC. In order to cover all of the A.F. amplifier bands, it therefore provides a wideband frequency response without a peak at any frequency. However, due to coupling capacitors at very low frequencies and shunt capacitors like stray capacitance at high frequencies, its frequency response is reduced.
Figure shows Transformer coupled Amplifier using Transistors. The output signal of first stage is coupled to the input of the next stage through an Impedance Matching Transformer
This type of Coupling is used to match the Impedance between output an input Cascaded stage. Usually, it is used to match the larger output resistance of AF power Amplifier to a low Impedance load like Loudspeaker. As we know, Transformer blocks d.c, Providing d.c. Isolation between the two stages. Therefore, Transformer Coupling does not affect the Quiescent point of the next stage.
In Comparison to an RC coupled Amplifier, the Frequency response of a Transformer-coupled Amplifier is subpar. Its inter winding Capacitances and leakage Inductance prevent the Amplifier from Amplifying signals of various Frequencies equally. The Coupling between the Transformer’s inter Windings may cause Resonance at a particular Frequency, which causes the Amplifier to produce very high gain at that Frequency. We can achieve Resonance at any desired RF Frequency by Connecting Shunting Capacitors to each Transformer winding. Tuned voltage Amplifiers are what these Amplifiers are known as.
These provide high gain at the desired of Frequency, i.e. they amplify Selective Frequencies. For this reason, the Transformer-coupled Amplifiers are used in radio and TV Receivers for Amplifying RF signals. As d.c. resistance of the Transformer winding is very low, almost all d.c. voltage applied by Vcc is available at the Collector. Due to the absence of Collector resistance it Eliminates Unnecessary power loss in the Resistor.
Figure depicts a Transistor-based direct coupled Amplifier. The input of the following stage is directly connected to the first stage’s output signal. Because of this direct Coupling, the first stage’s Quiescent d.c. Collector current can pass through the base of the second stage and change the Biassing conditions there.
Due to absence of RC Components, Frequency response is good but at higher Frequencies Shunting Capacitors such as stray Capacitances reduce gain of the Amplifier.
The Collector current and voltage of Transistors are affected by temperature changes in Transistor Parameters like VBE and. These changes show up at the base of the Subsequent stage due to direct Coupling, which also affects the output. Drift is a serious issue in direct coupled Amplifiers because it causes such an Unintended change in the output.