# Principles behind waveform selector circuit

by anandan111
Tags: circuit, principles, selector, waveform
 P: 14 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 Attached Thumbnails
 HW Helper Thanks PF Gold P: 4,916 What is the waveform of the 1st stage (the signal generator)?
P: 14
 Quote by rude man What is the waveform of the 1st stage (the signal generator)?
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.

 HW Helper Thanks PF Gold P: 4,916 Principles behind waveform selector circuit 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?
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P: 5,496
 Quote by anandan111 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.
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?
P: 14
 Quote by 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?
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?

 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?
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.

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

 HW Helper Thanks P: 5,496 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?
P: 14
 Quote by 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?
I will try to run the simulation with PSPICE...time to brush up my skills.
 P: 14 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 Attached Thumbnails
 P: 14 switch 4 - sine wave Attached Thumbnails
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P: 5,496
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?

Quote by rude man
What is the waveform of the 1st stage (the signal generator)?

 Quote by anandan111 The waveform is a sinusoidal 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.
P: 14
 Quote by 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?
The frequency of the square wave is 1 kHz.

 Rude man might be displeased to see the input now revealed as a square-wave!
My apologies as i was not familiar with stage 1 earlier and hence I thought the input was a sinusoidal wave.

 It could be useful to see the waveform at the junction of C7 and C4.
See attached image ^^

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

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P: 5,496
 Quote by anandan111 How do I calculate the theoretical output voltage for the sine wave? (Switch 4 from the left).
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?
P: 14
 Quote by 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?
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?
 HW Helper Thanks P: 5,496 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. hfe is the current gain of the transistor, collector current ÷ base current.
P: 14
 Quote by 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. hfe is the current gain of the transistor, collector current ÷ base current.
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?

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