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Biasing and Decoupling Op Amps Single Supply

Sam Sattel

Everyday-App-Note-Series

Everyday App Note: How to Bias and Decouple Op Amps in Single Supply Applications

Battery-powered op amp applications have a bit of a problem, they rely on a single power source. This setup often requires additional passive components that add complexity to an electronics project. If not properly designed, a single supply application can also have some serious instability problems. Today’s Everyday App Note from Analog Devices will teach you how to properly bias and decouple op amps in these applications.

Biasing Techniques Discussed

The technique most commonly used by designers is using a pair of resistors that acts as the divider and bias, as shown in the image below. Using this technique comes with a serious problem though, as it eliminates the useful Power Supply Rejection (PSR) feature in the op amp.

unstable-circuit-supply

(Image source)

To reintroduce power supply rejection into this design you can decouple the biasing network from the supply with the help of a capacitor as shown below. This capacitor will restore some of the PSR that was lost in the resistor biasing.

single-supply-decoupled-amplifier-circuit

(Image source)

Another way to provide biasing for a single supply application is through the use of a Zener diode regulator as shown below. The capacitor, CN, will block any noise from feeding into the op amp from the diode.

You need to carefully choose a Zener diode that has an operating voltage close to VS/2. The resistor chosen in this setup also needs to provide enough current to the Zener to operate at its stable rated voltage.

linear-voltage-regulator

(Image source)

Lastly, there’s also the option of using a linear voltage regulator for op amp circuits operating at the new 3.3V standard as shown below. This setup offers lower temperature changes and noise than a Zener diode setup does.

linear-voltage-regulator

(Image source)

Download This App Note Now

This application note discusses the details of these four biasing methods and the tradeoffs when using each. By the end of the reading you should have a solid understanding of which technique is right for your single supply application. The rest of the app note covers some of the general concerns related to op amp biasing, including:

  • How to handle noise issues in op amp applications with a low noise amplifier.
  • How to design a single supply application with an accurate turn-on time.
  • How to introduce headroom limitations for low voltage op amp applications.

Ready to get your single supply application done right? Download the Biasing and Decoupling Op Amps in Single Supply Applications app note now!

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