Principles behind waveform selector circuit

anandan111

1. The problem statement, all variables and given/known data
Hi i have a practical project that i am working on. I am working on a waveform selector circuit. My circuit is part two of three circuits. The first circuit is a signal generator followed by the waveform selector and then a filter rectifier. I am required to produce a report which explains how this circuit works. I need to explain the function of the circuit and individual components.
Kindly see attached for the schematic

3. The attempt
I know the circuit produces four different outputs as I have tested my circuit using a breadboard in the laboratory. I would get a sine wave, square wave, triangular wave and sawtooth wave. So far I have managed to achieve the required output. My sine wave is having a little problem such that it is a very sharp curve at the peak which almost looks like a triangular wave. Not too sure why though.

My question is that what is the required background reading on the topics which I need to know how the circuit functions? I am interested to know how the circuit components (transistors, capacitors, resistors) interact to produce different waveforms.

Your help is very much appreciated
Thank you

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rude man

What is the waveform of the 1st stage (the signal generator)?

anandan111

The waveform is a sinusoidal wave
The first stage is the testing of individual circuits so i tested by plugging in a (literal) signal generator and then selected the sine wave. I could then change waveforms by switching on different switches.
Thanks.

rude man

Hard to see what to make of this circuit. Where did you get it?


The top output should be a pure sine wave = 1/11 of your signal generator amplitude. After that it's hard to say. In any case I would not recommend this circuit as a source of sine, square and triangular waveforms. The transistor looks like a diode at its base, therefore introducing a nonlinearity to the network. If I had to predict its performance I would put it on a software simulator like PSPICE.

Again - who came up with this thing?

NascentOxygen

The subscripts are indistinct. Can you write out what they are?

Undoubtedly, the circuit is limited to a range of frequencies over which it gives acceptable performance. At what frequency did you test your breadboard?

To assist you in arriving at an explanation, you should accurately sketch the waveform at every node in this circuit, and noting the voltage level at each point of interest on the graphs. Having done this to help yourself, it will be helpful if you then post it here.

Is the square-wave at the base or at the collector?

anandan111

well our lecturer came up with this circuit as an introduction to our course which also forms our first project. I think the reason why he came up with this is because there is a section in our report where we have to point out the weaknesses of the circuits and suggest improvements to the circuits.
when you mention the top output. Did you meant switching on the first switch from the top? Also how did you get 1/11 of the signal generator amplitude? If I assume you were referring to the first switch then by potential divider rule, the voltage is across R8 right?
i got 47/57 :/
Also when we talk about performance, in what aspects should we discuss it? Like do we need to mention frequency response, voltage and current levels?

Thank you for your generous reply.
Well i tested my circuit at about 1000 Hz on a (literal) signal generator. And our next lab session will be in February and during our first testing session I forgotten to note down which switch corresponds to which waveform since at that time all of us were focusing on getting our circuit to work properly on the breadboard. Nevertheless, I will sketch the waveforms and record their respect voltage levels at every node at my next lab session.
Thank you for your generous reply

p.s. I have reuploaded another attachment and this is about the clearest I can get since my document does not show clearly too.

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NascentOxygen

You could discover a lot about this with software simulation, and would be better prepared for the practical class. In software you could vary the amplitude and frequency of the input sinusoid, looking for the "best" output waveforms. You could also vary the Beta of the transistor. Datasheets list the BC548 as having a wide spread of current gain, maybe there would be an optimal value?

anandan111

Thank you very much for your advice =D
I will try to run the simulation with PSPICE...time to brush up my skills.

anandan111

hey guys! this is my output waveform for all 4 switches...switch 4 is in my post below...( cause i am only allowed to upload a max of 3 files only... )

switch 1 starts from the potential divider, then the first RC low pass filter..then another RC low pass filter and switch 4 is the output across the common emitter amplifier

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anandan111

switch 4 - sine wave

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NascentOxygen

There's not much to say. It's just a series of RC stages and the waveform gets filtered as it moves from left to right. What frequency is that square wave?

Rude man might be displeased to see the input now revealed as a square-wave!

It could be useful to see the waveform at the junction of C7 and C4.

anandan111

The frequency of the square wave is 1 kHz.

My apologies as i was not familiar with stage 1 earlier and hence I thought the input was a sinusoidal wave.

See attached image ^^


Question:
1. How do I calculate the theoretical output voltage for the sine wave? (Switch 4 from the left).

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NascentOxygen

I think that would be beyond you. Do you think you are expected to be able to do that?

Do you know Laplace Transforms? Can you analyze the small signal or hybrid Pi model for a common emitter amplifier?

anandan111

well i don't know Laplace but i know phasor analysis and complex numbers.
I know how to convert that model into a hybrid parameter model and calculate the Av, Ai, Zi, Zo. looking at datasheets, the BC548 transistor hfe value has a min of 110 and max of 800.
May I know what this means?

NascentOxygen

It's too complicated for you. I'd say the best you could do is to analyze just the transistor stage, showing that Q1 together with R6, R9, R11 and C2 form a low-pass filter having a certain corner frequency and a certain low-frequency gain, and then plot it.

h_fe is the current gain of the transistor, collector current ÷ base current.

anandan111

Thank you NascentOxygen! You have been a great help =D
I will just calculate the Voltage gain through hybrid parameter model in my report

How do I calculate the corner frequency for the active low pass filter?