# Problem with understanding oscillator

#### Robesim

Joined May 1, 2017
134
Hi all, i'm a mechanical engineer and learning electronics by myself. I had a few electrical courses at school but i want to take it further. I read a few electronic study books and also learned myself working with LTSpice. I have some difficulties understanding oscillators. I know an oscillator consists of 3 basic parts:

1 Amplifier
2 Wave shaping network
3 Positive feedback path.

A Can someone please have a look at the oscillator and point out the 3 basic parts of the oscillator.
B What is the role of capacitors C2 and C3 .

Thanks

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#### Wendy

Joined Mar 24, 2008
22,360
I'm going to assume this is not homework and help you eith a piece of the puzzle, I tend to treat electronics problems as puzzles I will redraw the prat of the circuti I recognize and go through it with you. This does not make me an expert or correct, but it will give us a starting point in th dicusion. I will be backas it takes time to draw coherent schematics.

#### Wendy

Joined Mar 24, 2008
22,360
OK first clue This is actually a common base amplifier, I has moderate gain is a non inverting amp withmedium input and outpu impedance The thing that throw peop is is is drawn like a common emitter BJT amp.But note the base is groundedAC wise like a common base configuration I did d not finish calculating the components because I was in a hurry. If you have any questions so far please ask.

Figure 1

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#### AlbertHall

Joined Jun 4, 2014
11,214
xox

#### Robesim

Joined May 1, 2017
134
OK first clue This is actually a common base amplifier, I has moderate gain is a non inverting amp withmedium input and outpu impedance The thing that throw peop is is is drawn like a common emitter BJT amp.But note the base is groundedAC wise like a common base configuration I did d not finish calculating the components because I was in a hurry. If you have any questions so far please ask.

View attachment 137014
Why is your schematic a common base amplifier??
It looks like a common emitter amplifier. See the link below.

https://www.elprocus.com/common-emitter-amplifier-circuit-working/

I have a schematic attached about how i see oscillators. A control box which controls a switch. A sensor is giving the control box information about the status of the output. Based on that information, the control box turns the switch on or off. So in the oscillator schematic i first posted, transistor Q1 acts as a switch and Q2 is sensing the output and turns Q1 off. If Q1 is off then Q2 will also be off and the whole process starts again by recharching capacitor C1. Capacitor C1 is the wave shaping component i think (not sure), but what do C2 and C3 do??

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#### Wendy

Joined Mar 24, 2008
22,360
The base is firmly ground With a capitor, it is being held at a steady DCvalue, No signal can appear at that point Common Base amplifier are usually used fo RF signals because internal capacitances in the BJT ar minimized.Creatinga flat freq response over a wide rang of frequencies. Lets look at a textbook common base BJT Amp...

Figure 2

I will answer you other question in time, but fist you have to under stand some basic concepts fist,I'm trying to explain thes concepts fist. You will note the amp in figure2 requires several DC voltages. By changing the biasing a bit the need for several power supplies is eliminated.

The important thing about common base amp is the are non inverting which dramatically affect feedback requirements for oscillation.

I will draw a textbook CB oscillator which I believe will help make you problem clearer..

Long story short a simple cap between the collector and emitter is positive feedback. After that comes the frequency selection components

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#### AlbertHall

Joined Jun 4, 2014
11,214
but what do C2 and C3 do??
Look at the arrangement of C2, C3 and R3, R4 and compare with the circuits from my link abpve.

#### Robesim

Joined May 1, 2017
134
Look at the arrangement of C2, C3 and R3, R4 and compare with the circuits from my link abpve.
Yes, the arrangement is exactly the same. I'm getting somewhere now. R3, R4, C2 and C3 control the frequency of the oscillation and they form RC filters. I already studied passive filter circuits, but i didn't look at R3, R4, C2 and C3 that way. Good article about oscillators. I will study it through and get back to you with more questions. Thanks

#### Wendy

Joined Mar 24, 2008
22,360
Loooking at yor schematic again I am following a wild goose.

Looking at it again it is a basic multivibrator

When Q1 is on it turns Q2 on and when Q2 is on it turns Q2 off. Basically digital in operatio.During startup C3 anC1 have no charge turning both transistors off C1is charged through R1 eventuallturningQ1 onI'm not sure of the mrchanism but C2 is also part of an RC network, WhenQ1 is on it will discharge through R4 WhenQ1 turns off.I'm tring to figure out witch RC network does ehat , as in controllin which transistors on/off time. Sorry if I am not more help.

#### AlbertHall

Joined Jun 4, 2014
11,214
Loooking at yor schematic again I am following a wild goose.

Looking at it again it is a basic multivibrator
I think you were right first time. When first switched on, C1 will begin to charge. When there is enough voltage to turn on Q2, we have a non-inverting common base amplifier with a wien bridge feedback network so it starts to oscillate. As C1 continues to charge and the amplitude of oscillation increases the positive peak of the Q2 emitter voltage will begin to turn on Q1 thus limiting the C1 voltage and oscillation amplitude.

#### Robesim

Joined May 1, 2017
134
I think you were right first time. When first switched on, C1 will begin to charge. When there is enough voltage to turn on Q2, we have a non-inverting common base amplifier with a wien bridge feedback network so it starts to oscillate. As C1 continues to charge and the amplitude of oscillation increases the positive peak of the Q2 emitter voltage will begin to turn on Q1 thus limiting the C1 voltage and oscillation amplitude.
At the beginning C1 will charge. When there is enough voltage Q1 will turn on. When Q1 is on C2 will discharge, C3 will charge, Q2 will turn on and drain the charge of C1. Q1 will turn off and then Q2 will turn off and then C1 will charge again and the cycle repeats itself.

So in this case capacitor C1 is the wave shaping device, Q1 is the amplifier and R3, R4, C2, C3 and Q2 form the positive feedback network.
Am i correct??

#### Wendy

Joined Mar 24, 2008
22,360
This is a digital circuit.Q1 and Q2 are either on or off controlled by the RC networks. multivibrators are oscillators but are in their own class generally of the relaxation types.. To understand this circuit you must follow the capacitors chargingand how they discharge when the transistors switch states. When I first saw this circuit I was fooled into thinking it was a linear type circuit by the vague similarity to what I said earlier.The fact the transistor acquire opposite on/off states is significant.

I will be completely honest with you, I do not totally understand this circuit. I se e glimpses how it should operates by visualizing some of th steady state states it hits It is definitely not a sine wave generator. more likely it will produce square wave due to the transistor switching back and forth.

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#### Wendy

Joined Mar 24, 2008
22,360

Let me give a try at analysis.When The circuit turns on Q1 turns ondue to R1 chargingC1, This charges C3 through R2. When C3 is charged Q2 tuns on foces Q1 to turn of Q2 stays one until C3 discharges throughR3 until there is no longer enough chargeto keep Q2 on at which pointQ2 switches off starting the cyle again fot Q1.

R3 is labeled th load so I would not count it orC2 as part of the oscillator as a whole.

Note: If this was homework I have cheated you out og solving the problem. which does not benifite you and makes me an ass , We have a home work forum just for cases like this.

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#### AlbertHall

Joined Jun 4, 2014
11,214
Here's the kicker. My simulation of this circuit doesn't oscillate! It rings for a few cycles that's all.

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#### Robesim

Joined May 1, 2017
134
Here's the kicker. My simulation of this circuit doesn't oscillate! It rings for a few cycles that's all.
View attachment 137064
Here's the kicker. My simulation of this circuit doesn't oscillate! It rings for a few cycles that's all.
View attachment 137064
Then you must be doing something wrong. Look at R4 and turn "output" into a value. I think that's the problem. This is a working oscillator. I designed the part with the transistors, resistors and C1, but it didn't work. A member of this forum added the capacitors C2, c3 and the resistor R3 and then it worked perfect.

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#### Robesim

Joined May 1, 2017
134

Let me give a try at analysis.When The circuit turns on Q1 turns ondue to R1 chargingC1, This charges C3 through R2. When C3 is charged Q2 tuns on foces Q1 to turn of Q2 stays one until C3 discharges throughR3 until there is no longer enough chargeto keep Q2 on at which pointQ2 switches off starting the cyle again fot Q1.

R3 is labeled th load so I would not count it orC2 as part of the oscillator as a whole.

Note: If this was homework I have cheated you out og solving the problem. which does not benifite you and makes me an ass , We have a home work forum just for cases like this.
This is no homework. I am a mechanical engineer, trying hard to understand electronics. When i studied hydraulics i realized electronics can't be that complicated. I had a problem with the two capacitors. i know that C3 keeps Q2 on for a while to fully drain C1, but what is the purpose of c2??

#### RichardO

Joined May 4, 2013
2,271
Here's the kicker. My simulation of this circuit doesn't oscillate! It rings for a few cycles that's all.
View attachment 137064
Then you must be doing something wrong. Look at R4 and turn "output" into a value. I think that's the problem. This is a working oscillator. I designed the part with the transistors, resistors and C1, but it didn't work. A member of this forum added the capacitors C2, c3 and the resistor R3 and then it worked perfect.
Changing R3 in AlbertHall's circuit to 100 ohms does cause the simulation to work as a relaxation oscillator. I have to admit that I don't know why.

#### Robesim

Joined May 1, 2017
134
Changing R3 in AlbertHall's circuit to 100 ohms does cause the simulation to work as a relaxation oscillator. I have to admit that I don't know why.
The oscillator doesn't work if R3 is 100 ohms.

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#### mtonge

Joined Apr 19, 2016
93
I too was baffled by this oscillator. I posted my questions in this thread. This forum was very helpful.