Coupled amplifiers

 Coupling methods
Used for inter stage coupling of two amplifier stages are: 1. RC coupling method : Coupling employing two resistors mid a capacitor as coupling elements is called RC coupling. One resistor acts as a load resistor of first stage and a capacitor feeds the signal to the following stage. Second resistor is used to provide u-discharging, path for the DC charge stored in the capacitor. This type of circuit remains Tree from ‘direct pickups of radio waves defect.
2. Impedance coupling method : It is a modified form of RC coupling in which an inductive load is used in place of resistive load. The overall gain of an impedance coupled amplifier is higher than that of a RC coupled amplifier.
3. Transformer coupling method : It is a coupling employing a transformer as a coupling element. The primary of the transformer acts as an inductive load of the first stage and secondary acts as a signal source for the following stage. In this method, two amplifier stages remain isolated with each oilier to the DC source.
4. Direct coupling method : It is a coupling without using any coupling element. This method of coupling is distortionless and gives a uniform response over wide frequency range.

Direct coupling amplifier

FEED BACK AMPLIFIER

If the feedback signal is of opposite polarity to the input signal, then the negative feedback results. While
negative feedback results in reduced overall voltage gain, following improvements are obtained, :
(a)Higher input impedance
(b) Better stabilized voltage gain
(c) Improved frequency response
(d) Lower output impedance
(e) Reduced noise
(f) More linear operation
Types of Feedback Connections.
(i) Voltage -series feedback
(ii) Voltage - shunt feedback °
(iii) Current - series feedback
(iv)Current - shunt feedback.

AMPLIFIER CLASSES

One method generally employed to categorize amplifier is by class which basically represent the mount of output signal varies over one cycle of "operation for a full cycle of input signal.

1.Class A : The amplifier circuits which falls into this category are linear as these operate in a small

region in the middle of the load line.

2. Class B : This amplifier circuit provides an output signal varying over one-half the input signal cycle.

Class ration creates a very distorted output Bee ton of the input takes place for only _ 180° of the output signal swing. Power efficiency ‘of class-B is more than that of Class-A.

Classification of Amplifiers

 According to frequency range.

1. DC(Direct coupled ) amplifiers: From zero frequency dc on wards.

2.Audio frequency amplifiers : 20Hz to 20kHz

3.Video Frequency amplifiers : up to few MHz

4.Radio frequency (RF) amplifiers : From a few kHz to hundreds of MHz.

5. Ultra high frequency (UHF) and Microwave amplifiers: Upto hundred or thousands of MHz,

According to Method of operation.

The position of the zero signal (quiescent) operating point and the extent of characteristic curves being used determine the category of operation. Thus amplifiers (using either BJT or FET) may be  classified as class A, class AB, class B or class C  amplifiers as per following definition.

(1).Class A amplifiers.: In these operating point and the input signal magnitude are so selected that the output circuit current (collector current or drain current) flows all the time (for full cycle of the a.c. signal). A class A amplifier operates essentially over the linear portion of characteristic curves of the device.

(ii) Class B amplifier. In-these operating point is placed at an extreme end of its characteristic curve with the result that zero signal current (or zero signal voltage) is almost zero. Hence zero signal output power is very small. Witha sinusoidal input signal voltage, amplification takes place for only half the cycle. Thus if output circuit quiescent current is zero, on application of signal, the output current remains zero for half the cycle and flows for the remaining half cycle.

(iii) Class AB amplifier. In these operation lies in between class A and class B operations. Thus in a class AB amplifier, output current flows for more than half but less than the complete cycle of input sinusoidal signal.

(iv) Class C amplifier. In these operating point is so chosen that the output current flows for less than one-half of the input sinusoidal signal cycle.

FIELD EFFECT TRANSISTORS (FET)

Field effect transistor. Operation depends on the flow of majority carriers only. So , itis a unipolar device. No minority carrier current.  In N channel FET : Width of space between P regions is controlled by varying gate voltage (Controlled by gate-to- source potential),

Advantages :

1. Easy / Simpler to make

2. High input impedance (Mega Q)

3. High power gain

4.Less noisy

5.No offset voltage at zero drain current

RECTIFIERS

It is conversion of an Alternating current into Unidirectional current,

Types of rectifiers are :

1.Haf wave Rectifier

2. Full wave Rectifier

3. Bridge Rectifier

Half-wave rectifier circuit :

It provides DC output for positive half cycles only: Maximum rectification efficiency of a half  wave

Full-wave rectifier;

It provides DC output  for both half cycles of Ac.

Advantages:

1. High efficiency.

2.Ordinary filter circuit is sufficient 

3.Provides more output current

Bridge Rectifier

It is a transformer less full wave rectifier. it requires 4 diodes.

Advantages:

Full-wave rectifier (two diodes type) requires a transformer with centre-tapped secondary, whereas, bridge rectifier does not require such transformer. 

1, Diode valve rectifier

Diode valve is a two electrode vacuum tube which consists of a cathode and an anode. When cathode is heated up by passing a current through it filament, it starts to emit electrons. Anode emitted electrons if it is made positive with re to cathode. If AC is applied to anode, then a current will flow for each positive half cycle only. In this way, AC will be converted into DC. Silicon diode is better tor the rectification of AC.

2. Metal rectifier:

similarly 

So, electric current can flow easily from a sharp edged point towards a flat surface even across a thin layer of an insulation.

Type of Metal rectifiers:

(1) Selenium rectifier

(2) Copper oxide rectifier