1. Subject Code : 331101 Name Of Subject : Basic Electronics Name of Unit : OSCILLATORS & AMPLIFIERS. Topic :Hartely Oscillator & Colpitt oscillator By :Mr. Harekrushna Avaiya

2. Circuit Explanation :-  Q is NPN transistor.  R1 and R2 are biasing resistor.  RE is the resistance connected in the emitter.  CE is the bypass capacitor.  (L1+L2)C form the tank circuit.  RFC is the radio frequency chocke which allows d.c to pass through and offers high impedance to a.c.  Oscillation energy developed across the choke is coupled to tank circuit by coupling capacitor C2. By:-Harekrushna Avaiya

3. Working:-  When power supply is given,collector current flows through the choke.  A.C. energy developed in the collector circuit given to tank circuit through coupling capacitor Cc2.  Voltage developed across L2 is coupled to the base through coupling capacitor Cc1..

5.  Q is NPN transistor.  R1 and R2 are biasing resistor.  RE is the resistance connected in the emitter.  CE is the bypass capacitor.  (C1+C2)L form the tank circuit.  RFC is the radio frequency choke which allows d.c to pass through and offers high impedance to a.c.  Oscillation energy developed across the choke is coupled to tank circuit by coupling capacitor C2.

6.  When power supply is given, collector current flows through the choke.  A.C. energy developed in the collector circuit given comes to tank circuit through coupling capacitor Cc2.  Voltage developed across C2 is coupled to the base through coupling capacitor Cc1..

8.  CONSTRUCTION:-  SCR is 4-layer,3-junction and 3-terminal PNPN semiconductor device.  Here, three terminal is given as,  -Anode,  -Cathode &  -Gate

9.  Working of SCR can be easily understand by two transistor analogy.  If we divide the middle two layers in two parts, then construction becomes as shown in figure.  Here, one PNP transistor & other one is NPN transistor.

10.  As shown in figure, collector of first transistor is connected to base of second transistor and the collector of second transistor is connected to the base of first transistor.

11.  When anode is connected to positive supply and cathode is connected to the negative,and gate is not connected to any supply, both the transistor are in OFF state, they offer very high resistance and whole voltage is dropped in the SCR and output is zero.  When a positive pulse is given to the gate, base current IB1 flows. So, collector current IC1 flows. But the collector current IC1 of transistor T1 is the base current IB2 of transistor T2. So, the IB2 flows and as a result, collector current IC1 flows. Now, this current IC2 flows through the base of transistor T1.So,the collector current IC1 increases. This is cumulative process. As a result a large current is conducted by the device in very short time, the resistance of the device decrease too much.  So,load can be control by very small positive pulse in SCR.