R A Q ’ s S p e c i a l A d v e r t i s i n g S e c t i o n Rarely Asked Questions Single Supply Amplifiers—They Sound Simple…are they? Q. Running Running rail-to-rail rail-to-railopop amps amps on a supply sounds like a like winning onsingle a single supply sounds a combination, but what drawbacks winning combination, but what will I encounter using amplifiers drawbacks willwhen I encounter when like this? using amplifiers like this? A. A. Single-supply Single-supply and and rail-to-rail rail-to-rail outputs outputs are a great combination, are a great combination, but but there there are are a a few few parameters parameters that that warrant warrant a a secsecond ond look. look. Your Your question question doesn’t doesn’t specify specify whether you are talking about whether you are talking about single-supsingle-supply ply amplifiers amplifiers (a (a particular particular class class of of ampliamplifiers) or running a traditional op fiers) or running a traditional op amp amp on on single single supply; supply; we’ll we’ll discuss discuss both both cases. cases. By By definition, definition, a a true true “single-supply” “single-supply” op op amp operates on one amp operates on one supply, supply, and and the the input input common-mode common-mode voltage voltage range range of of the amplifier includes the negative the amplifier includes the negative supply suprail. Note thatthat even though the the input of ply rail. Note even though input the amplifier can go to the negative rail of the amplifier may be able to go to the and beyond doesn’t mean the output negative rail itand beyond it doesn’t mean can. I’ll say more about that when talk the output can. I’ll say more aboutwe that about rail-to-rail outputs. when we talk about rail-to-rail outputs. Any Any amplifier amplifier can can be be run run on on a a single single supply. Op amps don’t have supply. Op amps don’t have a a ground ground pin pin and and are are equally equally happy happy when when run run on on a a bipolar bipolar supply supply as as they they are are with with a a single single supply. supply. Additional Additional bias bias circuitry circuitry is is required required when when running running an an amplifier amplifier in in this this configuration, configuration, however. however. As As a a result, result, the the amplifier’s amplifier’s performance performance may may suffer suffer slightly slightly in the following areas: lower bandwidth, in the following areas: lower bandwidth, degraded degraded power power supply supply rejection rejection (PSR), (PSR), and higher noise. and higher noise. The The term term rail-to-rail rail-to-rail output output is is a a misnomer. misnomer. While the amplifier output can While the amplifier output can get get close close to to the the supply supply rails, rails, it it never never quite quite gets gets there. there. In In bipolar bipolar amplifiers, amplifiers, the the rail-torail-torail output stage is typically rail output stage is typically a a common common emitter; emitter; therefore therefore the the closest closest the the output output can get to the rails is a saturated can get to the rails is a saturated transistransistor tor drop drop Vcesat. Vcesat. The The value value of of Vcesat Vcesat is is Contributing Writer John Ardizzoni is an Application Engineer FPO at Analog Devices in the High Speed Linear group. John has been with Analog Devices since 2002, he received his BSEE from Merrimack College in N. An- dependant on the amount of load current dependant the amount For of load current delivered byon the amplifier. low curdelivered by the amplifier. For low currents, the output can come to within tens rents, the output to within of millivolts to thecan rail.come For higher cur-tens of millivolts to the rail. For higher rents Vcesat can approach 0.5 V orcurmore. rents can approach 0.5 V oron-chip more. SomeVcesat new amplifiers now feature Some new amplifiers now feature on-chip charge pumps to make up the Vcesat charge pumps to upto the Vcesat drop, allowing themake output swing all the drop, allowing the output to truly swing way to the rail. all the way to the rail. An amplifier output swinging close to An output swinging close to the amplifier supply rail may appear fine when the supply rail may appear fine when measured on an oscilloscope, but a measured on an oscilloscope, a network analyzer may reveal a but different network analyzer may reveal a different result. As the output swings closer to result. Asthe theoutput outputtransistors swings closer to the rails, no longer the rails, the output transistors no longer operate in the linear region. As a result, operate linear region. As a result, distortioninisthe introduced. Distortion can distortion is introduced. Distortion occur several hundreds of millivoltscan away occur several millivolts away from the rails.hundreds Thereforeofwhen possible, from the rails. Therefore when possible, try design in a little extra headroom try design in a this littlewill extra headroom from the rails; help improve the from the rails; this will help improve the amplifier distortion performance. amplifier distortion performance. We’ve only skimmed the surface on these We’ve skimmedapplication the surfaceand on these topics. only For in-depth topics. For in-depth application and product information, visit our website or product information, visit our website or click on the link below. click on the link below. dover, MA and has over 28 years experience in the electronics industry. Have a question involving a perplexing or unusual analog problem? Submit your question to: raq@reedbusiness.com For Analog Devices’ Technical Support, Call 800-AnalogD SPONSORED BY To Learn More About Single Supply XXX XXXAmplifiers XXX Go to: http://rbi.ims.ca/XXX-100 http://rbi.ims.ca/5726-100 d e s i g n n e w s 1 0 . 0 6 . 0 8 [ w w w. d e s i g n n e w s . c o m ] ##