# Simple question for the gurus

#### typhune

Joined Apr 19, 2011
2
I'm in the midst of learning circuit design, and trying to get some practical experience, taking a simple circuit and walking through what each piece does to understand it.

So I picked up the Velleman MK136 Super ear kit. Schematic here: http://www.vellemanusa.com/downloads/0/minikits/manuals/manual_mk136.pdf

I'm trying to understand what purpose R5/R6 (Top near opamp) perform. Looks like a voltage divider circuit, but the sine wave from the mic side is also passing through it.

Is this effectively adding 2.25v to the mic circuit (Assuming a 4.5v supply)? Is there a name for this type of circuit?

#### Wendy

Joined Mar 24, 2008
21,906
OK, here is the schematic, cleaned up a bit...

This is called bias. It allows the op amp to operate in the middle of the power supply voltage. The AC signal rides on the DC. This concept is used on transistors too, as well as many other types of linear circuits.

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#### typhune

Joined Apr 19, 2011
2
Looks like the attachment didn't come through.

Joined Apr 30, 2011
1,570
It is a voltage divider and it is adding 2.25V (nominal) to the AC coupled signal. You were right on those points. Bill called it bias which is also correct.

Put this all together and you get "voltage divider bias". The All About Circuits ebook has a section on biasing but it's incomplete so you might look at the Wikipedia article: http://en.wikipedia.org/wiki/Bipolar_transistor_biasing.

#### wmodavis

Joined Oct 23, 2010
739
Marsden is right on, its providing DC bias to the op-amp input.

#### Ron H

Joined Apr 14, 2005
7,014
NE5532 is not specified to work with a supply voltage of Vcc-Vee less than 10V. It probably won't work well, if at all, at 4.5V.

Joined Apr 30, 2011
1,570
NE5532 is not specified to work with a supply voltage of Vcc-Vee less than 10V. It probably won't work well, if at all, at 4.5V.
That's what I thought but On Semiconductor shows theirs operating <3V single supply. http://www.onsemi.com/pub_link/Collateral/NE5532-D.PDF and Velleman's been getting away with it. It'd be tempting to try the circuit on 9V with a few component values changed of course.

#### Ron H

Joined Apr 14, 2005
7,014
That's what I thought but On Semiconductor shows theirs operating <3V single supply. http://www.onsemi.com/pub_link/Collateral/NE5532-D.PDF and Velleman's been getting away with it. It'd be tempting to try the circuit on 9V with a few component values changed of course.
I couldn't find that in the datasheet. What page is it on?
The only similar thing I could find was that it is spec'ed down to ±3V power supplies.

Joined Apr 30, 2011
1,570
Page 5, figures 7 & 8 infer operation at low supply voltages in the X axis.

#### BillB3857

Joined Feb 28, 2009
2,501
Also, title page states, "
Features
• Small-Signal Bandwidth: 10 MHz
• Output Drive Capability: 600 , 10 VRMS
• Input Noise Voltage: 5.0 nV Hz (Typical)
• DC Voltage Gain: 50000
• AC Voltage Gain: 2200 at 10 kHz
• Power Bandwidth: 140 kHz
• Slew Rate: 9.0 V/s
Large Supply Voltage Range: +/- 3.0 to +/-20 V
• Compensated for Unity Gain
• Pb−Free Packages are Available