TRANSISTOR TUNED AMPLIFIERS

Inroduction  Sometimes it is desired that an amplifier should amplify either a single frequency or a narrow band of frequencies. For instance, radio and television transmission are carried on specific radio frequency assigned to the broadcasting stations.

 To achieve this, the simple resistive load in the collector is replaced by a tuned LC circuit whose impedance strongly depends upon frequency. Such a tuned circuit becomes very selective and amplifies very strongly signals of resonant frequency and a narrow band on either side. Therefore, the use of the tuned LC circuit in conjunction with a transistor makes possible the selection and efficient amplification of particular desired frequency. Such an amplifier is called a tuned amplifier.

 Series tuned circuit. It is essentially a series LC circuit. The most important characteristic of this circuit is that at some frequency (called resonant frequency), the inductive reactance and capacitive reactance become equal, resulting in minimum circuit impedance and maximum circuit current. Under such conditions, the circuit is said to be in series resonance. At series resonance:

(i)Resonant frequency Where L and C are in Henery and Farad respectively.

(ii) Circuit impedance = R, the circuit resistance. (iii) Circuit current (iv) Q of coil Q of capacitor Q of circuit (v) Bandwidth of circuit

 Tuned Amplifiers. A tuned amplifier uses a tuned LC load in the collector instead of a resistive load. A tuned LC permits three principal advantages. First, it enables to select one frequency (i.e. resonant frequency) out of a number of frequencies present. Second, a tuned amplifier is always operated in class C mode for higher collector efficiency. The tuned LC load nullifies the distortion introduced due to class C operation. Third, by adjusting the coupling between the load and tank circuit, we can achieve impedance matching. The results in the maximum transfer of power to the load.

 Tuned amplifiers find wide application in electronics. For example, radio and television receives use tuned amplifiers to select one radio frequency from the many being broadcast. They are also used in other forms of communication such as a radar, sonar and telemetry.

CHAPTER 14: Amplifier with Negative Feedback BASIC ELECTRONICS SEPT

Introduction  A practical amplifier has a gain of nearly one million i.e. its output is one million times the input. Consequently, even a casual disturbance at the input will appear in the amplified form in the output. There is strong tendency in amplifiers to introduce hum due to sudden temperature changes or stray electric and magnetic fields.

 Therefore, every high gain amplifier tends to give noise along with the signal in the output. The noise in the output of an amplifier is undesirable and must be kept to as small a level as possible.  The noise level in amplifiers can be reduced considerably by the use of negative feedback i.e. by injecting a fraction of the output in phase opposition to the input signal.

Negative Feedback in Amplifiers  When a fraction of output energy (voltage or current) of an amplifier is fed back “in phase opposition” to the input, it is called negative feedback. A negative feedback amplifier has two parts viz. an amplifier and a feed circuit. The feedback circuit usually consist of resistor and returns a fraction of output energy to the input.

 Consider a negative feedback amplifier with the following particulars:  A= gain without feedback  m= feedback fractions  A fb = gain with negative feedback

It may be seen that the gain of the amplifier without feedback is A. but when negative feedback is applied, the gain is reduced by the factor 1+Am. The factor 1+Am is generally called “sacrifice factor” because the gain of the amplifier is sacrificed by this amount to improve the quality of the amplifier circuit.

Advantages of the Negative Feedback  Although negative feedback in an amplifier reduces the gain, yet the benefits derived thereof are many. Some of the advantages of negative feedback in amplifiers are:

i. It increases the gain stability i.e. gain becomes independent of changes in temperature, variations in transistor parameters and frequency. ii. It reduces the non-linear distortion. It can be easily proved that with negative feedback, non-linear distortion in amplifiers is reduced by a factor 1+Am. iii. It improves the frequency response of the amplifier becomes substantially constant over a wide range of signal frequency.

iv. Voltage feedback increases the input impedance and decreases the output impedance of the amplifier such a changes is profitable in practice as the amplifier can then serve the purpose of impedance matching. v. Current feedbacks reduces the input impedance and increases the output impedance and increases the output impedance. vi. Negative feedback reduces the phase-shift in a amplifier.

Feedback fraction (m)

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