Input Bias Resistor OpAmp NE5532 - Page 1
Forum section: electronics / beginners

author: Amoebius
author_rank: Contributor
timestamp: January 07, 2023, 11:38:17 am
content: Hi,I have a question regarding OpAmps.In the picture I included is a circuit i found on the net for a Non Inverting Audio Amplifier.My understanding is that the 20K R2 restistor is there for biasing the input current of the OpAmp. When this is the case, should it not be more like 2K, since the inverting input is looking at R4 and R3, 2K and 20K in parallel, so the value would be below 2K.I tested the ciruit with R2 set to 2K and 20K, both works fine...Could someone explain this to me?Greetings,Dirk

author: jasonRF
author_rank: Regular Contributor
timestamp: January 07, 2023, 03:40:20 pm
content: Quote from: Amoebius on January 07, 2023, 11:38:17 amBoth should work fine, and it really will not matter in your case since your output is AC coupled. If you were designing a circuit and wanted to keep the output DC offset very small, then you would probably pick 1.8k for that resistor, since it is very close to 2k || 20k. That would give both inputs the same resistance to 0V, so when both inputs draw their similar bias currents (into the bases of the BJTs at the inputs) they will end up at about the same voltage.EDIT: I should add that in your case the value of that resistor effects the cutoff frequency of the high-pass filter created by C2 and R2. Since you probably do not care at all about the exact DC level of the opamp output, if you wanted to lower the cutoff frequency you could increase that resistor value to 100k and still should be fine.jason

author: Roehrenonkel
author_rank: Frequent Contributor
timestamp: January 07, 2023, 03:49:06 pm
content: Hi Dirk,you want to match the resistances at the inputs for low-drift dc-applications.Since you are ac-coupling you don't have to care about that.Higher R2 --> smaller C-input.Lower R2 --> less (Resistor-) noise and higher Cut-off-frequency.Ciao4now

author: BillyO
author_rank: Super Contributor
timestamp: January 07, 2023, 05:37:58 pm
content: Try removing it and see what happens.

author: Doctorandus_P
author_rank: Super Contributor
timestamp: January 07, 2023, 06:28:22 pm
content: If the goal is to reduce offsets due to bias current, then R2 should be equal to the parallel resistance of R1 and R4. But it is not important here. Offset due to the bias current though a small resistor is small, and both input and output are AC coupled.20k is a reasonable input impedance for an audio amplifier. It also forms a high pass filter with the input capacitor, and this capacitor would have to be bigger if the resistor is smaller (for the same RC timeconstant). You always need some kind of resistor in there, because the bias current from the opamp input needs a DC path to somewhere. without a DC current path, the opamp simply does not work at all. But the resistor can be almost anything between a few hundred ohm (depends on what is driving that input) and a few Mega Ohm (this will increase offset voltage a bit, but is filtered out again at the output. High value resistors also generate more noise.At the moment the cutoff frequency is:1/(2*pi*10e-6*1e3) = 16 Hz.if you lower R2 to 2k, then the input capacitor would have to be 100uF (for the same frequency response) and this is outside the normal range of high quality film capacitors. You would need multiple of them in parallel, which increases cost and PCB area.

author: Terry Bites
author_rank: Super Contributor
timestamp: January 07, 2023, 07:41:00 pm
content: R2 provides the necessary path for bias currents too, but that is not its primary purpose.R2, C2 set the circuit input's high pass (-3dB) corner. R2 is chosen to be >> than the source impedance.The feedback nework is used to set the gain. Again the network also allows bias currents a DC path. In this case via 0V and the opamp's output. The values in the feedback network are not necessarily chosen to suit bias current requirements. The bias currents are usually very small compared to the signal currents. This is one reason why you won't see high value reistors used so much in feedback networks. Hence the popularity of values in the 10K area.As ponted out, equalising the reistance seen by both opamp inputs will reduce offset errors and drift . Normally not a big worry in an AC coupled circuit.

author: Amoebius
author_rank: Contributor
timestamp: January 07, 2023, 08:37:02 pm
content: Ok, thanks for the answers, that explains it well.One thing, wouldn't the -3dB point be1/(2pi*0,000010*20000)?

author: Terry Bites
author_rank: Super Contributor
timestamp: January 08, 2023, 05:41:54 pm
content: Input high pass corner is 1/(2*pi() R2*C2)Output high pass corner is 1/(2*pi() R1*C1)Usually you want these figures close to each other.If you need a low pass response as well, shunt R4 with a cap Cx.Low pass corner 1/(2*pi() R4*Cx). Save on your calculator buttons wear and tear:www.muzique.com/schem/filter.htmIt is wise to limit the bandwidth (BW) with hp and lp of any amp. You dont want to amplify any spurious out of band stuff if you can help it.Fb resistor noise. En (RMS) = √4kTR·BW

author: TimFox
author_rank: Super Contributor
timestamp: January 08, 2023, 06:09:03 pm
content: Limiting the low-frequency response at the input of an audio amplifier is a good idea to reduce out-of-band response to "thumps" and "rumble", and to avoid some kinds of transient IM distortion, but generally the high-pass filter at the output should cut-off at a lower frequency than that at the input. The overall low-frequency response is then determined by the input filter.

author: Amoebius
author_rank: Contributor
timestamp: January 08, 2023, 07:46:37 pm
content: Ah, good to know, thank you.