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Principles behind waveform selector circuit 
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#1
Jan2113, 09:44 AM

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 


#2
Jan2113, 11:29 AM

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



#3
Jan2213, 12:16 AM

P: 14

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. 


#4
Jan2213, 10:42 AM

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PF Gold
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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? 


#5
Jan2213, 11:11 PM

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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 squarewave at the base or at the collector? 


#6
Jan2313, 02:29 AM

P: 14

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. 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. 


#7
Jan2313, 07:58 PM

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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?



#8
Jan2513, 10:01 AM

P: 14

I will try to run the simulation with PSPICE...time to brush up my skills. 


#9
Apr3013, 07:45 AM

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 


#10
Apr3013, 07:45 AM

P: 14

switch 4  sine wave



#11
Apr3013, 09:08 PM

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


#12
May113, 12:40 AM

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


#13
May113, 01:03 AM

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


#14
May113, 01:52 AM

P: 14

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? 


#15
May113, 06:38 AM

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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 lowpass filter having a certain corner frequency and a certain lowfrequency gain, and then plot it.
h_{fe} is the current gain of the transistor, collector current ÷ base current. 


#16
May213, 01:24 AM

P: 14

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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