In the previous article we got some information about oscillators definition, sinusoidal oscillators, essential of transistor oscillators, so today we are going to discuss the different types of transistor oscillators. So let’s start the topic.
If the tank and feedback circuit are properly connected a transistor can work as an oscillator to produce continuous discontinuous oscillations of any desired frequency.
All oscillators under different names have the similar function, that is, they produce output continuously without stopping. However, the major difference between these oscillators lies in the method by which energy is supplied to the tank circuit to compensate for the losses.
Following are the transistor oscillators commonly use at various places in electronic circuits :
- Tuned collector oscillator
- Colpitts oscillator
- Hartley oscillator
- Phase shift oscillator
- Wien bridge oscillator
- Crystal oscillator
See also this : Sinusoidal oscillator – What is Barkhausen criterion – Essentials of transistor oscillator
What is tuned collector oscillator?
The circuit consists of tuned circuit L1 – C1 in the collector side transistor. So the frequency of oscillation depends on the values of L1 and C1 and is given by
f = 1 / (2π √L1 C1) ………………(1)
The feedback coil L2 in the base circuit is magnetically coupled to the tank circuit coil L1. The capacitor C provides low reactance path to the oscillation. The biasing is provided by voltage divider arrangement.
Circuit operation
When switch S is closed, the collector current starts increasing and charges the capacitor C1. When the capacitor fully charged it’s discharge through coil L1, setting up oscillations of the frequency determined by exp (1).
These oscillation induce some voltage in coil L2 by mutual induction. The voltage across L2 applied between base and emitter and appears in the amplified form in the collector circuit.
The number of turns of L2 and the coupling between L1 and L2 are adjust such that the oscillations across L2 are amplified to a level sufficient to supply losses to the tank circuit.
Due to the transformer action, a phase shift of 180° is create between the voltages of L1 and L2. Another phase shift of 180° occurs between the base emitter and collector circuits due to the transistor properties.
As a result the energy feedback to the tank circuit is in phase with the generated oscillations.
What is Colpitts oscillator? – Colpitts oscillators
It uses two capacitors and a common inductor placed across L and the center of the two capacitors is tapped. The tank circuit is made up of C1, C2 and L. So the frequency of oscillation is determined by the values of C1, C2 and L and is given by
f = 1/ ( 2π √LCT ) ………….(1)
Where, CT = C1C2 / C1 + C2
Colpitts oscillator uses a pair of tapped capacitors (C1 and C2) and an inductor L to produce a 180° phase shift in the feedback network. So here C1 – C2 – L is the feedback network.
The output voltage from the amplifier appears across C1 and the feedback voltage is develops across C2.
Circuit operation
When the circuit is on, the capacitors C1 and C2 are charge. The capacitors discharge through L, and setting up oscillations of the frequency determined by exp (1).
The output voltage of the amplifier appears across C1 and the feed back voltage develops across C2. The voltage across C2 is 180° out of phase with the voltage developed across C1 (Vout) as per the feedback circuit diagram, you can see in the figure below.
A phase shift of 180° is produces by the transistor. And another phase shift of 180° is produce by the C1 – C2 voltage divider. So In this way, the feedback is precisely phased to produce continuous damped oscillations.
See also this : What is transistor audio power amplifier?
What is the feedback fraction for Colpitts oscillator?
What is Hartley oscillator? – Hartley oscillators
The Hartley oscillator is an electronic oscillator which is frequency of this oscillator is determine by the tuned circuit. The tuned circuit consists of capacitor and inductor hence it is called L-C oscillator.
Hartley oscillator is similar to Colpitts oscillator with minor modifications. Instead of using tapped capacitors, two inductors L1 and L2 are placed across a common capacitor C and the centers of the inductors is tapped.
The tank circuit is made up of L1 , L2 and C. So the frequency of oscillations is determined by the values of L1 , L2 and C and is given by
f = 1/ ( 2π √CLT ) ………………..(1)
Where, LT = L1 + L2 + 2M
Here, M = Mutual inductance between L1 and L2
Note that L1 − L2 − C is also a feedback network which produces a phase shift of 180°.
Operation of Hartley oscillator
When the circuit is on, the capacitor is charge. When this capacitor is fully charged, it discharges through the coils L1 and L2 , setting up oscillations of the frequency determined by exp (1).
The output voltage of the amplifier appears across L1 and feedback voltage across L2. The voltage across L2 is 180° out of phase with the voltage developed across L1 (Vout) as per the feedback circuit diagram, you can see in the figure below.
A phase shift of 180° is produces by the transistor. And another phase shift of 180° is produce by the L1 − L2 voltage divider. So in this way, the feedback is precisely phased to produce continuous damped oscillations.
What is the feedback fraction of Hartley oscillator?
Feedback fraction mv : In Hartley oscillator, the feedback voltage is across L2 and output voltage is across L1.
Therefore, Feedback fraction mv = Vf / Vout
Applications of Hartley oscillator
- In radio receiver this oscillator is use.
- It is use in the radio frequency in a range of the 30MHz.
What is phase shift oscillator? – phase shift oscillators
The circuit consists of a conventional single transistor amplifier and an RC phase shift network.
Phase shift network consists of three sections R1C1 , R2C2 , R3C3. At some particular frequency f0, the phase shift in each RC section is 60° , so that the total phase shift produce by RC network is 180°.
So the frequency of oscillations is given by,
f0 = 1 / (2π RC √6 )………………….(1)
Where, R1 = R2 = R3 = R
C1 = C2 = C3 = C
Remember : The phase shift oscillators are rarely use because they are extremely unstable.
Circuit operation
When the circuit is switch on, it produces oscillations of frequency as determine. The output E0 of the amplifier is feedback of the RC feedback network.
This network produces a phase shift of 180° and the voltage Ei appears at its output which is apply to the transistor amplifier. So the feedback fraction is m = Ei / Eo.
The phase shift of 180° is produces by the transistor amplifier. Another phase shift of 180° is produced by the RC network. As a result, the phase shift is 360° around the entire loop.
See also this : What are the different classification of power amplifiers?
Advantages of phase shift oscillator
- It does not require transformer or inductor.
- It can be use to produce a very low frequency.
- It provides good frequency stability.
Disadvantages of phase shift oscillator
- It is difficult for the circuit to start oscillation because the feedback is generally small.
- The circuit gives small output.
What is Wien bridge oscillator? – Wien bridge oscillators
The Wien bridge oscillator is use for all frequencies in the range of 10 Hz to about 1 MHz. It is essentially a two-stage amplifier with an R-C bridge circuit. The bridge circuit has arms R1C1 , R3C3 , R2C2 and a tungsten lamp LP.
Resistance R3 and LP are use to stabilise the amplitude of the output. Transistor T1 acts as an oscillator and amplifier while the other transistor T2 acts as an inverter (That is to produce of phase shift of 180°).
The circuit uses positive feedback through R1C1 , C2 R2 to the transistor T1. The negative feedback is through a voltage divider to the input of transistor T2.
The frequency of oscillations is determine by the series element R1C1and the parallel element R2C2 of the bridge.
So , f = 1 / (2π √R1 C1 R2 C2 )
If R1 = R2= R
and C1 = C2 = C
then, f = 1 / (2π √RC) ……………(1)
Operation of Wien bridge oscillator
When the circuit is start, the bridge circuit produces oscillations of the frequency as determined by equation (1). The two transistors produce a total phase shift of 360° to ensure proper positive feedback.
Negative feedback in the circuit ensures constant output and it is achieve by the temperature sensitive tungsten lamp LP. So Its resistance increases with current.
The amplitude of output tend to increase, more current will provide more negative feedback. As a result output returns to original value. The function of LP is to maintain the voltage.
Advantages of Wien bridge oscillator
- It gives constant output.
- The overall gain is high.
- The frequency of oscillation can be easily change by using a potential meter.
Disadvantages of Wien bridge oscillator
- The circuit requires two transistors and a large number of components.
- It cannot generate very high frequency.
See also this : Transistor Configuration
What is a transistor crystal oscillator?
This is a Colpitts oscillator that has been modify to act as a crystal oscillator. The only change is the addition of a crystal (Y) to the feedback network. So the crystal will act as a parallel-tuned circuit.
As you can see in this circuit instead of resonance due to L and (C1 + C2) we have parallel resonance of the crystal. At parallel resonance, the impedance of the crystal is maximum. So it means that there is a maximum voltage drop across C1.
This in turn will allow maximum energy transfer through the feedback network on the fP. A phase shift of 180° is produced by the transistor. Again another phase shift of 180° is produced by the capacitor voltage divider.
This oscillators will oscillate only on fP. Even the smallest deviation from fP will cause the oscillator to act as an effective short. As a result we have a very stable oscillator.
Advantages of crystal oscillator
- It has a high order of frequency stability
- The Q-factor of the crystal is very high ( 10,000 as compare to about 100 of L-C tanks).
Disadvantages of crystal oscillator
- They are delicate and consequently can only be use in low power circuit.
- The frequency of oscillations cannot be change appreciably.
See also this : What is transistor in electronics
I hope this information (Different types of transistor oscillators) will help you. Thank you