single stage rc coupled amplifier experiment pdf

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RC Coupled Amplifier Circuit Working, Types and Frequency Response

February 3, 2020 By WatElectronics

RC coupled amplifier is a basic type of amplifier with the various stages present in it. In other words, if we describe RC coupled amplifier we can say that an amplifier that consists of resistors and capacitors which acts a voltage divides and couplers to form multiple/single stage for better amplification. These are the basic circuits that are present in the various types of electronic equipment especially in RF signal or other communication devices as it helps in improving the signal strength through amplification.

What is an RC Coupled Amplifier?

An amplifier with the multiple stages based on the necessary levels of amplification can be defined as an RC coupled amplifier. It can be connected in any transistor configurations based on the efficiency of the system. It is widely used in audio and video communication devices.

RC Coupled Amplifier Circuit Diagram

The RC coupled Amplifier circuit may consists of various types of transistor configurations connected with the resistors and the capacitors. The configurations can be of the common emitter, common collector or the common base.  In this article, we are discussing a single stage and two stages RC coupled amplifiers.

Single Stage RC Coupled Amplifier

Single-stage RC coupled amplifier can be termed as a preamplification circuit. Because these circuits are designed to improve the strength of the weak signals for the further amplification process.

This type of amplifier in the single-stage consists of the resistors R1, R2 and the capacitor. Here the capacitor is utilized so that it can provide the smoothing effect to the signal. It is capable of preventing the DC signals to enter and allow the AC one’s. The resistors R1 and the R2 that is connected at the input side to see to  that the proper biasing voltage reaches the transistor so that the amplification process starts.

Further, the resistors that are present at the emitter and the collector are responsible to maintain the voltage drop to a minimum of up to 50 percent. With the help of the capacitors that are present at the emitter and the collector to avoid negative feedback because the stability is a major concern. In this way, the single-stage amplifier acts as the pre-amplifier.

Two-Stage RC Coupled Amplifiers

Two stage RC Coupled Amplifier

After the processing of the signal in the first stage the output of the first stage, the amplifier is connected to the second stage as the input. This made possible with the coupling capacitor present at the output of the first stage of the amplifier. One can observe after processing occurs at the second stage the output signal obtained has the increased gain factor in it and it is in phase with the input signal. In this way, the two stages RC coupled amplifier works.

 Frequency Response of RC Coupled Amplifier

Bandwidth can be considered to be the major factor during the amplification of the signals. This can even influence the gain factor. When the curve or the graph is plotted in between the values of the gain and the frequency of the amplifier the resultant curve obtained is known as the frequency response of that particular amplifier. The bandwidth is the difference between the high and low frequency in which the gain remains at its higher value. It could be seen in the below graph that at a lower frequency the gain is low because the reactance of the capacitor is low at lower frequency and similarly at higher frequency the capacitor CE acts a short cirucuit an so again the gain is low.

Frequency Response of the RC Coupled Amplifier

Experiment:

This experiment of the RC coupled amplifier can be performed with the basic resistors and the capacitors with the transistor circuit. So let us discuss the experiment in detail.

Aim:  the purpose of the experiment is to obtain the frequency response and the gain of voltage for the RC coupled amplifier.

Apparatus Required:

The apparatus required for the experiment is as follows:

• Transistor (BC107) – 2
• Resistors (3.3K, 33K, 330 ohms, 1K) – 2
• Capacitors (100 micro F, 10 micro F) – 3 and 2

The experiment performed here is consists of a two-stage RC coupled amplifier. This is very efficient in terms of gain. The output of the first stage is coupled to the second stage as input through the capacitor. The capacitor is also known for blocking the DC signals and providing the smoothing effect to the signals obtained at the output.

As the gain obtained at the two stages will be different this gain values obtained are plotted on the curve with the frequency values. The final gain will be calculated with the product of the individual gains obtained at each stage. The curve obtained from this is known as the frequency response of the amplifier.

• The input signal is applied with the function generator.
• The values of the voltage are observed and noted at the first stage and the second stage.
• Based on the values noted the gains at the individual stages are calculated.
• This is continued for multiple frequencies.
• Finally, the graph is plotted in between the gain values and the multiple frequencies applied to the circuit.

  Observations

The observations of the output voltage & voltage gain for applied frequency can be done by using the above experiment and note down the values in the tabular form.

  Precautions

• The type of transistor used and its terminals must be identified correctly for providing proper connection.
• The connections established must be tight.

In this way, the gain value of the voltage and the frequency response of the RC coupled amplifier is calculated.

Applications of RC Coupled Amplifier

The applications of the RC coupled amplifier are as follows:

• Based on the frequency response of the RC coupled amplifier the gain remains constant to the maximum extent for the frequencies applied. Hence this makes this amplifier to be suitable for amplification of the audio signals.
• These amplifiers are preferred as Preamplifiers.
• This amplifier is also widely used in the amplification of the voltage signals.

In this way the these are proven to be very advantageous because of its low cost and the frequency response obtained. The presence of the components called resistor and the capacitors in this circuit makes it of low cost. This RC coupled amplifer can be made a multistaged one. For an even staged amplifier the output would be in phase with the input but for a odd stage amplifier like 3 or 5 the output would be in phase shift with the input. But as the circuit becomes older day by day it tends to get easily affected by the noise an so there would a loss in the quality of the signal. However, it is very advantageous in terms of the amplification of audio signals. But other than this can you tell what the major drawback of this amplifier is?

RC Coupled Amplifier: What is it? (Working Principle & Applications)

What is a rc coupled amplifier.

A Resistance Capacitance (RC) Coupled Amplifier is basically a multi-stage amplifier circuit extensively used in electronic circuits. Here the individual stages of the amplifier are connected together using a resistor – capacitor combination due to which it bears its name as RC Coupled.

In this kind of amplifier, the input signal applied at the base of the transistor in stage 1 (Q 1 ) is amplified and appears at its collector terminal with a phase-shift of 180 o .

Advantages of RC Coupled Amplifier

Disadvantages of rc coupled amplifier, applications of rc coupled amplifier, leave a comment cancel reply.

• Engineering & Technology
• Electrical Engineering

Experiment No. -1 Aim: To Plot the Frequency Response of a single

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M Dash Foundation: C Cube Learning

RC coupled amplifier and its low frequency response, Lecture-XXIII.

Analog systems and applications — lecture – XXIII.

RC coupled single stage amplifier and its low frequency response, Lecture – 23.

This article belongs to a series of lectures on analog electronics, the paper goes by the name “Analog Systems and Applications” for the physics honors degree class. All lectures of this series will be found  here . This is the 23rd lecture of the series. The lecture was delivered on 29th March 2018.

In our last two lectures we discussed in much detail the case of the single stage common emitter RC coupled amplifier. There we discussed the elements of the circuits (such as why a given capacitor is required and so on), determined the currents in various components (such as base and collector) and eventually calculated the current, voltage and power gain of such an amplifier. In doing so we employed the hybrid or equivalent circuit analysis method.

Today we would like to delve on the subject a little deeper and see what exactly is the response of a single stage RC amplifier to a given frequency. Let us remember the diagram that represented the circuit for this amplifier (taken from first of the last two lecture, linked above).

The common emitter circuit single stage amplifier. It consists of specially placed resistors for voltage divider type bias and capacitors for achieving desired actions.

Frequency response of a single stage RC amplifier.

Lets define three frequency regions.

• Low : if the frequency is less than 50 Hz.
• Medium : If the frequency is between 50 Hz to 20 kHz.
• High : If the frequency is larger than 20 kHz.

Mid frequency range:

The voltage gain is independent of frequency in the mid frequency zone. lets see whats it like with the low frequency

Low frequency range:

Series reactance of the coupling capacitor [ 1/(ωC C ) ] is no more negligible. The emitter capacitor ( C E ) and the input capacitor ( C in ) are assumed to have large values. As a result their reactance can be safely neglected and that means they provide a conducting path (like a short).

Low-frequency single stage RC coupled amplifier.

Lets now draw the AC equivalent circuit for the low-frequency single stage RC coupled amplifier.

The low frequency RC coupled amplifier in its hybrid or equivalent circuit form. Note how there is an extra capacitor now compared to the circuit we drew for it for mid-frequency in our last two lectures. Now this comes in between the two resistors which were earlier dealt as parallel giving an effective AC resistance R ac . Now the equations are going to be different because of this altered scenario.

Lets apply Kirchhoff’s voltage law in the input circuit: V i – h re V ce = i B h ie . eq n 1

Lets apply Kirchhoff’s current law at node C: h fe i B + h oe V o + V ce /R C + jωC C (V ce – V o ) = 0 . eq n 2

The same AC current passes through C C and R L (as they are in series) ⇒ jωC C (V ce – V o ) = V o /R L . eq n 3

Now we can compare the voltage gain at low frequency to that at the mid frequency region.

We see that voltage gain at low frequencies is less than voltage gain at mid frequencies. Also this further decreases with decrease in frequency.

Lower cutoff frequency f 1 is defined as that value of frequency at which voltage gain is 1/√2 times mid frequency voltage gain. Thus power gain is 1/2 times mid frequency power gain, at this value of low frequency signal. i.e. at f = f 1 , | A V,l / A V,m | = 1/√2 and P V,l / P V,m = 1/2 .

Then at f = f 1 , | A V,l / A V,m | = 1/√2 = 0.707 ≡ 70.7 % according to our conjecture. Overall phase shift of voltage gain in low frequency range is given as: φ l = 180 0 + θ l = 180 0 + tan -1 (f l /f) . At f = f 1 , φ l = 180 0 + 45 0 = 225 0 .

We see that the effective resistance R eff for lower half-power frequency f = f 1 is the series addition of resistances on both sides of the capacitance C C , i.e. R C and R L .

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3 responses to “RC coupled amplifier and its low frequency response, Lecture-XXIII.”

[…] we can find the voltage gain etc as before (in the case of medium and small frequency, see linked […]

[…] They must be something in the electronics industry. In our last 5 lectures (lectures 21, 22, 23, 24 and 25) we discussed in good detail the frequency response of the single stage RC coupled […]

[…] feedback, see these articles how amplifier voltage gain has been calculated for various amplifiers: low, medium, […]

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The resistance-capacitance coupling is, in short termed as RC coupling. This is the mostly used coupling technique in amplifiers.

Construction of a Two-stage RC Coupled Amplifier

The constructional details of a two-stage RC coupled transistor amplifier circuit are as follows. The two stage amplifier circuit has two transistors, connected in CE configuration and a common power supply V CC is used. The potential divider network R 1 and R 2 and the resistor R e form the biasing and stabilization network. The emitter by-pass capacitor C e offers a low reactance path to the signal.

The resistor R L is used as a load impedance. The input capacitor C in present at the initial stage of the amplifier couples AC signal to the base of the transistor. The capacitor C C is the coupling capacitor that connects two stages and prevents DC interference between the stages and controls the shift of operating point. The figure below shows the circuit diagram of RC coupled amplifier.

Operation of RC Coupled Amplifier

When an AC input signal is applied to the base of first transistor, it gets amplified and appears at the collector load R L which is then passed through the coupling capacitor C C to the next stage. This becomes the input of the next stage, whose amplified output again appears across its collector load. Thus the signal is amplified in stage by stage action.

The important point that has to be noted here is that the total gain is less than the product of the gains of individual stages. This is because when a second stage is made to follow the first stage, the effective load resistance of the first stage is reduced due to the shunting effect of the input resistance of the second stage. Hence, in a multistage amplifier, only the gain of the last stage remains unchanged.

As we consider a two stage amplifier here, the output phase is same as input. Because the phase reversal is done two times by the two stage CE configured amplifier circuit.

Frequency Response of RC Coupled Amplifier

Frequency response curve is a graph that indicates the relationship between voltage gain and function of frequency. The frequency response of a RC coupled amplifier is as shown in the following graph.

From the above graph, it is understood that the frequency rolls off or decreases for the frequencies below 50Hz and for the frequencies above 20 KHz. whereas the voltage gain for the range of frequencies between 50Hz and 20 KHz is constant.

We know that,

$$X_C = \frac{1}{2 \pi f_c}$$

It means that the capacitive reactance is inversely proportional to the frequency.

At Low frequencies (i.e. below 50 Hz)

The capacitive reactance is inversely proportional to the frequency. At low frequencies, the reactance is quite high. The reactance of input capacitor C in and the coupling capacitor C C are so high that only small part of the input signal is allowed. The reactance of the emitter by pass capacitor C E is also very high during low frequencies. Hence it cannot shunt the emitter resistance effectively. With all these factors, the voltage gain rolls off at low frequencies.

At High frequencies (i.e. above 20 KHz)

Again considering the same point, we know that the capacitive reactance is low at high frequencies. So, a capacitor behaves as a short circuit, at high frequencies. As a result of this, the loading effect of the next stage increases, which reduces the voltage gain. Along with this, as the capacitance of emitter diode decreases, it increases the base current of the transistor due to which the current gain (β) reduces. Hence the voltage gain rolls off at high frequencies.

At Mid-frequencies (i.e. 50 Hz to 20 KHz)

The voltage gain of the capacitors is maintained constant in this range of frequencies, as shown in figure. If the frequency increases, the reactance of the capacitor C C decreases which tends to increase the gain. But this lower capacitance reactive increases the loading effect of the next stage by which there is a reduction in gain.

Due to these two factors, the gain is maintained constant.

Advantages of RC Coupled Amplifier

The following are the advantages of RC coupled amplifier.

The frequency response of RC amplifier provides constant gain over a wide frequency range, hence most suitable for audio applications.

The circuit is simple and has lower cost because it employs resistors and capacitors which are cheap.

It becomes more compact with the upgrading technology.

Disadvantages of RC Coupled Amplifier

The following are the disadvantages of RC coupled amplifier.

The voltage and power gain are low because of the effective load resistance.

They become noisy with age.

Due to poor impedance matching, power transfer will be low.

Applications of RC Coupled Amplifier

The following are the applications of RC coupled amplifier.

They have excellent audio fidelity over a wide range of frequency.

Widely used as Voltage amplifiers

Due to poor impedance matching, RC coupling is rarely used in the final stages.