tuned collectortuned collector oscillator

8/10/2019 Tuned CollectorTuned Collector Oscillator

1/4

Tuned collectorTuned collector oscillator.

Tuned collector oscillation is a type of transistor LC oscillator where the tuned

circuit (tank) consists of a transformer and a capacitor is connected in the collector

circuit of the transistor. Tuned collector oscillator is of course the simplest and the

basic type of LC oscillators. The tuned circuit connected at the collector circuit

behaves like a purely resistive load at resonance and determines the oscillator

frequency.The common applications of tuned collector oscillator are RF oscillator

circuits, mixers, frequency demodulators, signal generators etc. The circuit diagram

of a conventional tuned collector oscillator is shown in the figure below.

Circuit diagram.

In the circuit diagram resistor R1

and R2 forms a voltage divider bias for the transistor. Re is the emitter resistor

which is meant for thermal stability. It also limits the collector current of the

transistor. Ce is the emitter by-pass capacitor. The job of Ce is to by-pass the

amplified oscillations. If Ce is not there, the amplified AC oscillations will drop

across Re and will add on to the base-emitter voltage (Vbe) of the transistor and this

will alter the DC biasing conditions. Capacitor C1 and primary of the transformer L1forms the tank circuit. C2 is the by-pass capacitor for resistor R2.

Working of the tuned collector oscillator.

When the power supply is switched ON, the transistor starts conducting and the

capacitor C1 starts charging. When the capacitor is fully charged, it starts

discharging through the primary coil L1. When the capacitor is fully discharged, the
http://www.circuitstoday.com/wp-content/uploads/2009/10/tuned-collector-oscillator.png


2/4

energy stored in the capacitor as electrostatic field will be moved to the inductor as

electromagnetic field. Now there will be no more voltage across the capacitor to keep

the current through the coil starts to collapse. In order to oppose this the coil L1

generates a back emf (by electromagnetic induction) and this back emf charges the

capacitor again. Then capacitor discharges through the coil and the cycle is repeated.

This charging and discharging sets up a series of oscillations in the tank circuit.

The oscillations produced in the tank circuit is fed back to the base of transistor Q1

by the secondary coil by inductive coupling. The amount of feedback can be adjusted

by varying the turns ratio of the transformer. The winding direction of the secondary

coil (L2) is in such a way that the voltage across it will be 180 phase opposite to that

of the voltage across primary (L1). Thus the feedback circuit produces a phase shift

of 180 and the transistor alone produces a phase shift of another 180. As a result a

total phase shift of 360 is obtained between input and output and it is a very

necessary condition for positive feedback and sustained oscillations.

The collector current of the transistor compensates the energy lost in the tank

circuit. This is done by taking a small amount of voltage from the tank circuit,

amplifying it and applying it back to the tank circuit. Capacitor C1 can be made

variable in variable frequency applications.

The frequency of oscillations of the tank circuit can be expressed using the following

equation:

F = 1/[2(L1C1)]Where Fis the frequency of oscillation. L1is the inductance of the transformer

primary and C1is the capacitance.

oscillator.

Tuned collector oscillation is a type of transistor LC oscillator where the tuned circuit (tank)

consists of a transformer and a capacitor is connected in the collector circuit of the

transistor. Tuned collector oscillator is of course the simplest and the basic type of LC

oscillators. The tuned circuit connected at the collector circuit behaves like a purely resistive

load at resonance and determines the oscillator frequency.The common applications of

tuned collector oscillator are RF oscillator circuits, mixers, frequency demodulators, signal

generators etc. The circuit diagram of a conventional tuned collector oscillator is shown in

the figure below.

Circuit diagram.


3/4

In the circuit diagram resistor R1 andR2 forms a voltage divider bias for the transistor. Re is the emitter resistor which is meant

for thermal stability. It also limits the collector current of the transistor. Ce is the emitter

by-pass capacitor. The job of Ce is to by-pass the amplified oscillations. If Ce is not there,

the amplified AC oscillations will drop across Re and will add on to the base-emitter voltage

(Vbe) of the transistor and this will alter the DC biasing conditions. Capacitor C1 and

primary of the transformer L1 forms the tank circuit. C2 is the by-pass capacitor for resistor

R2.

Working of the tuned collector oscillator.

When the power supply is switched ON, the transistor starts conducting and the capacitorC1 starts charging. When the capacitor is fully charged, it starts discharging through the

primary coil L1. When the capacitor is fully discharged, the energy stored in the capacitor as

electrostatic field will be moved to the inductor as electromagnetic field. Now there will be

no more voltage across the capacitor to keep the current through the coil starts to collapse.

In order to oppose this the coil L1 generates a back emf (by electromagnetic induction) and

this back emf charges the capacitor again. Then capacitor discharges through the coil and

the cycle is repeated. This charging and discharging sets up a series of oscillations in the

tank circuit.

The oscillations produced in the tank circuit is fed back to the base of transistor Q1 by thesecondary coil by inductive coupling. The amount of feedback can be adjusted by varying

the turns ratio of the transformer. The winding direction of the secondary coil (L2) is in

such a way that the voltage across it will be 180 phase opposite to that of the voltage across

primary (L1). Thus the feedback circuit produces a phase shift of 180 and the transistor

alone produces a phase shift of another 180. As a result a total phase shift of 360 is
http://www.circuitstoday.com/wp-content/uploads/2009/10/tuned-collector-oscillator.png


4/4

obtained between input and output and it is a very necessary condition for positive

feedback and sustained oscillations.

The collector current of the transistor compensates the energy lost in the tank circuit. This is

done by taking a small amount of voltage from the tank circuit, amplifying it and applying it

back to the tank circuit. Capacitor C1 can be made variable in variable frequencyapplications.

The frequency of oscillations of the tank circuit can be expressed using the following

equation:

F = 1/[2(L1C1)]

Where F is the frequency of oscillation. L1 is the inductance of the transformer primary

and C1is the capacitance.

tuned collectortuned collector oscillator

Documents