Use of Transistors as Amplifiers in Electronic Circuits !

Use of Transistors as Amplifiers in Electronic Circuits !
Use of Transistors as Amplifiers in Electronic Circuits !
Use of Transistors as Amplifiers in Electronic Circuits !
Use of Transistors as Amplifiers in Electronic Circuits !
Use of Transistors as Amplifiers in Electronic Circuits !
Use of Transistors as Amplifiers in Electronic Circuits !
Use of Transistors as Amplifiers in Electronic Circuits !
Use of Transistors as Amplifiers in Electronic Circuits !
Use of Transistors as Amplifiers in Electronic Circuits !
Use of Transistors as Amplifiers in Electronic Circuits !
Use of Transistors as Amplifiers in Electronic Circuits !
Use of Transistors as Amplifiers in Electronic Circuits !
Use of Transistors as Amplifiers in Electronic Circuits !
Use of Transistors as Amplifiers in Electronic Circuits !
Use of Transistors as Amplifiers in Electronic Circuits !
Use of Transistors as Amplifiers in Electronic Circuits !
Use of Transistors as Amplifiers in Electronic Circuits !
Use of Transistors as Amplifiers in Electronic Circuits !

 Why transistors can be used as voltage and current amplifiers. The output characteristic curves of a transistor's response in a common-emitter configuration illustrate the change in current in the collector IC with respect to the voltage across the collector and emitter VCE for a certain base current IB. The transfer characteristic, which relates IC to IB for a given collector voltage and the concept of current gain ΔIC / ΔIB was also demonstrated.

Use of Transistors as Amplifiers in Electronic Circuits !

The current flowing between the emitter and the collector of a transistor is much greater than that flowing between base and emitter. Thus a small base current is controlling the emitter collector current. The ratio of the two currents, IC / IB is constant, provided that the collector emitter voltage VCE is constant. Therefore, if the base current rises, so does collector current. This ratio is the current gain of the transistor and is given the symbol hfe. A fairly low-gain transistor might have a current gain of 20 to 50, while a high gain type may have a gain of 300 to 800 or more. The spread of values of hfe for any given transistor is quite large, even in transistors of the same type and batch. The principle explained above is a remineder of why transistors can be used as amplifiers.

Remember that the current in the sum of the collector and base currents, and the base current IB are very small compared to IE or IC

Use of Transistors as Amplifiers in Electronic Circuits !

Some other important parameters of the transistor are given by the following relationships:

Use of Transistors as Amplifiers in Electronic Circuits !

Although not expressed in the previous formula, it is important to consider that βdc varies with both collector current and temperature. The transistor data sheet usually specifies (hfe) at specific IC values. Even at fixed values of IC and temperature, βdc varies from device to device for a given transistor. You will see how to overcome these problems with the use of feedback in the following sections.

Other important mathematical expressions are used for DC analysis, e.g. for the configuration in the next figure:

Use of Transistors as Amplifiers in Electronic Circuits !

Differential Amplifiers:    

 

For instrumentation applications, there are many situations for which we would like to amplify a small difference between two signal levels and ignore any "common" level both inputs may share, usually noise, which is approximately equal for both inputs. The rejection of unwanted signals is commonly quantified by the common-mode rejection ratio (CMRR), which is the ratio of differential gain to common-mode gain. This rejection ;can be achieved by using a differential amplifier; an example of one way to achieve this is shown in the following illustration.

Use of Transistors as Amplifiers in Electronic Circuits !

Differential amplifiers (also called difference amplifiers) are so named because they amplify the difference between two inputs. The type of circuit on the UniTrain-I "Differential amplifiers" card consists of a pair of similar transistors in a circuit known as a long-tailed pair. In this, the emitters of both transistors are connected to the negative of the supply voltage via a feedback arrangement. The negative of the supply may be at 0 V (asymmetrical power supply) or at a negative voltage that is equal to the positive voltage from the supply (symmetrical power supply). The bases of the two transistors form the inputs and the collectors are connected via equal load resistors to the positive supply.

The circuit thus features two more or less identical amplifier circuits. When their inputs are identical, they both amplify to the same extent and there should be no difference in their outputs. We then have a balanced circuit. If the inputs are slightly different however, the two transistors conduct different amounts of current so that the voltage drop across one transistor rises while the voltage across the other decreases. This means that the voltage at one transistor's collector rises while the voltage at the other collector drops by the same amount. The output of the circuit is then measured between the two collectors.

In practice it is not possible to manufacture components so precisely that the two circuits are truly identical. Due to component tolerances, there are always slight differences that give rise to a small offset between the two amplifier circuits. The circuit is thus assembled so that it is possible to compensate for this offset. In a process called trimming the offset, a potentiometer is used to balance the feedback from the two amplifiers so that the two transistor circuits produce zero output when the inputs are equal.

Darlington Amplifiers​:

 

The r parameter βac is a major factor in determining the input resistance. The βac of the transistor limits the maximum achievable input resistance you can get from a given emitter-follower circuit. One way to boost the input resistance is to use a Darlington pair such as the one shown in the following figure. The collectors of two transistors are connected and the emitter of the first one drives the base of the second. Darlington pairs are widely used in audio power amplifiers, high-current motor switches and other power switching applications.

Use of Transistors as Amplifiers in Electronic Circuits !    

 

The emitter current of the first transistor becomes the base current of the second transistor. Therefore, the effective current gain of the Darlington pair is obtained by multiplying the Beta parameters of the two transistors, as shown in the formulae above.

Here is an example of a commercial Darlington pair, a BC517 NPN Darlington transistor and some of its parameters: 

Use of Transistors as Amplifiers in Electronic Circuits !

Biasing Amplifier Circuits​:

 

As shown in the previous section, a transistor must be properly biased in order to operate as an amplifier. The purpose of dc biasing is to establish a steady level of transistor current and voltage called the DC operating point or quiescent (Q) point. A DC operating point must be set so that signal variations at the input terminal are amplified and accurately reproduced at the output terminal. Biasing a transistor means basically establishing a certain current and voltage condition. This means, for example, that at the DC operating point, IC and VCE have specified values. The following figure shows conditions of proper and improper biasing of an amplifier. Incorrect biasing can cause distortion in the output because of cut-off or saturation of the transistor under certain input signal conditions. That is, if the input signal is too large for the Q point location and is driving the transistor into cut-off and/or saturation during a portion of the input cycle.

 Use of Transistors as Amplifiers in Electronic Circuits !

Use of Transistors as Amplifiers in Electronic Circuits !  

The transistor in the following figure is biased with variables VCC and VBB to obtain certain values of IB, IC, IE, and VCE. The collector characteristic curves explain what happens to IC and VCE for various values of IB. When IBincreases, IC increases, and VCE decreases. So as VBB is adjusted up or down, the DC operating point of the transistor moves along a sloping straight line, called the DC load line, connecting each Q point. Here, the DC load line intersects the VCE axis at 10 V, the point where VCE = VCC. This is the transistor cut-off point because IB and IC are ideally zero. The dc load line intersects the IC axis at 50 mA ideally. This is the transistor saturation point because IC is ideally at its maximum at the point where VCE = 0 and IC = VCC/RC.

 Use of Transistors as Amplifiers in Electronic Circuits !

Symmetrical Amplifiers​: 

 

Basic analysis of a symmetrical amplifier

When both inputs are grounded, the emitters are at -0.7 V. In the ideal case that the transistors are identical, the following applies:

Use of Transistors as Amplifiers in Electronic Circuits !

Since both collector currents and both collector resistors are equal (when the input voltage is zero), the following equation is true:

Use of Transistors as Amplifiers in Electronic Circuits !

If input 2 is grounded and a positive bias voltage is applied to input 1, the positive voltage on the base of T1 increases IC1 and raises the emitter voltage to

Use of Transistors as Amplifiers in Electronic Circuits !

This action reduces the forward bias (VBE) of T2 because its base is held at 0V, thus causing IC2 to decrease. The net result is that an increase in IC1 causes a decrease in VC1 and the decrease in IC2 causes an increase in VC2. The opposite would happen if we connected input 1 to ground and applied a positive input bias to T2.

Modes of Signal Operation:

Single-ended input. One input is grounded and the signal voltage is applied to only the other input. The signal appears inverted (and amplified in accordance with the voltage gain) at the output of the transistor to which it was applied and non-inverted at the output of the other one, because the emitter signal becomes an input to this latter transistor as the emitters are common.

Differential input. Two opposite-polarity (out-of-phase) signals are applied at the inputs. Each input affects the outputs. The operation can better be understood by tying one input to ground at a time, doing the analyses and superimposing both the resulting output signals.

Common-mode input. Two signal voltages of the same phase, frequency and amplitude are applied at the two inputs. Again, the basic operation can be understood by considering each input signal as acting alone. When the separate output signals are superimposed, they cancel, resulting in an output voltage of zero. This action is called common-mode rejection. Its importance lies in the situation when an unwanted signal appears at both differential amplifier inputs. This unwanted signal will not appear on the outputs to distort the desired signal. Common-mode signals (noise) generally are the result of the input lines picking up radiated frequencies from adjacent lines, the 50 Hz (60 Hz) mains or other sources.

The common-mode rejection ratio CMRR can also be calculated from the differential gain Av(d) to the common-mode gain, Acm. It can also be expressed in decibels.

      Use of Transistors as Amplifiers in Electronic Circuits !

In summary, desired signals appear on only one input or with opposite polarities on both input lines. These desired signals are amplified and appear at the outputs. 

Summary:
  • Apart from other common applications, transistors are used as signal amplifiers in electronic circuits due to the fact that the base current is linearly amplified at the collector current by the factor ßdc or hfe for a constant voltage VCE within the linear operating region.

 

  • Voltage amplification means that the voltage input is modified by a gain factor AV at the output,
    Use of Transistors as Amplifiers in Electronic Circuits !
    .
    It can also be expressed in decibels as
    Use of Transistors as Amplifiers in Electronic Circuits !.

 

  • Biasing implies setting a correct DC level in order to correctly activate the transistor as an amplifier when it is needed, and not drive it into cut-off and/or saturation modes. Common biasing configurations for amplifiers are common-emitter, common-collector, and common-base. For power amplifiers, it might be desirable, however, to drive the amplifier into saturation mode in order to avoid excessive power dissipation.

 

  • The DC load line is a visual means for observing the DC linear operating point and region of the transistor. The AC behaviour is different from the DC behaviour and, in order to calculate it, AC equivalent circuits involving transistor parameters are used.

 

  • Darlington amplifiers increase the current gain by means of an arrangement featuring two transistors in cascade.

 

  • Multi-stage transistor amplifiers are used to overcome gain and impedance limitations. Their gain can be calculated by either dividing the output voltage by the input voltage or by multiplying the gains of its stages Use of Transistors as Amplifiers in Electronic Circuits ! or in decibels,

         

Use of Transistors as Amplifiers in Electronic Circuits !
  • Multi-stage transistor amplifiers can be coupled either directly or with other passive electronic components. The effects of such coupling on the gain, impedance matching, biasing voltages for the following stages and frequency response ought to be considered.

 

  • The parameter ßdc or hfe is dependent on temperature and is different for every transistor. In order to provide a stable gain and to couple impedances in circuits, feedback is needed to provide an input which is dependant on the output. Gain reductions involving feedback need to be considered.

 

  • A differential amplifier provides a gain for a small difference between two signal levels and rejects common voltage levels (noise) to both inputs. This rejection is quantified by the common-mode rejection ratio (CMRR). This circuits should usually provide a means for trimming offsets and balancing the circuit.

 

  • Current sources should provide the same amount of current independent of the load. A voltage divider bias configuration with a zener diode to compensate for temperature dependence or an FET can provide a stable current source. 

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