EEVblog Electronics Community Forum
Electronics => Beginners => Topic started by: SilverHawk on October 13, 2016, 02:10:27 pm
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Hey guys, i was hoping you could help me with a few questions and correct me if im wrong
1) R1 and R2 are biasing resistors which shift the input ac signal by 7.5V right since the negative power pin is grounded( Vs is 15v) , my question is why is R3 necessary ?
2) Based on simulation, C4 makes the DC gain of the op amp to around 1, can anybody explain in simple terms or calculations to get this value
thanks.
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R3 is included because the reference voltage (7.5V) is bypassed by C2 to ground.
C4 allows the AC gain to be set by the feedback resistors. At DC, the feedback resistor to ground is effectively disconnected.
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Ok so mathematically gain = 1+ R5/R4 when its DC voltage, due to the capacitor Xc4->infinity therefore gain = 1
sorry but I still dont get why you need R3 , perhaps another way to explain it.
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Without R3 then the filter capacitor C2 shorts the input signal to ground. C2 prevents ripple on the supply voltage from being amplified.
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What about the function of R6C6 ? What happens if i dont have it in my circuit
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A lot of posters have explained why R3 is there to DC bias the +ve input to 7V5 but I'm going to say, try a thought experiment and think of R3 in terms of two extremes, short or open, if it were open then there would be no DC bias and if were short then C2 would give you an AC short to ground, R3C2 time constant defines the low frequency cutoff as well as R1 and R2 to a certain extent. R6C6 is a Zobel network (Bell Labs 1923) to compensate for the inductive load and keep the impedance seen by the amplifier output terminals fairly constant over frequency, it also damps back EMF from the loudspeaker. In RF engineering for example, if you were driving an inductive load, a transformer for example, you might put an RC network in parallel with the load to keep the impedance constant over the required working frequency range, sometimes called duplexers or diplexers. If your Zobel network wasn't there it might cause instability problems at high frequencies. In switched mode power supplies you will see series RC snubbers to suppress high voltage spikes caused by leakage inductance, in practice RCD snubbers are used because the diode makes them directional but that is a different topic. A good tip for young players, try to think of circuits in the frequency domain (spectrum analyzer) as well as the time domain (scope) it will give you a lot more insight.
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A lot of posters have explained why R3 is there to DC bias the +ve input to 7V5 but I'm going to say, try a thought experiment and think of R3 in terms of two extremes, short or open, if it were open then there would be no DC bias and if were short then C2 would give you an AC short to ground, R3C2 time constant defines the low frequency cutoff as well as R1 and R2 to a certain extent.
22k is not the ideal value for R3.
To minimise the effect of the bias currents on the offset voltage, the DC resistance between each input and the power supply rails should be as equal as possible. In this case, the inverting input is connected to the output of the amplifier (a very low impedance) via a 22k (R5) so the non-inverting input should also have 22k in series with half the power supply voltage. At first the non-inverting input may appear to have 22k in series with it but that doesn't take into account the impedance of the potential divider, which will be equal to R1 & R2 in parallel: 11k in this case.
The ideal value for R3 is 22k - 11k = 11k
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What about the function of R6C6 ? What happens if i dont have it in my circuit
Don't you have the datasheet? It tells you what happens when any part's value is wrong.