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Unformatted text preview: AN-937APPLICATION NOTEOne Technology Way P.O. Box 9106 Norwood, MA 02062-9106, U.S.A. Tel: 781.329.4700 Fax: 781.461.3113 www.analog.com Designing Amplifier Circuits: How to Avoid Common Problems by Charles Kitchin Rev. 0 | Page 1 of 8 INTRODUCTION When compared to assemblies of discrete semiconductors, modern operational amplifiers (op amps) and instrumenta- tion amplifiers (in-amps) provide great benefits to designers. Although there are many published articles on circuit applications, all too often, in the haste to assemble a circuit, basic issues are overlooked leading to a circuit that does not function as expected. This application note discusses the most common design problems and offers practical solutions. MISSING DC BIAS CURRENT RETURN PATH One of the most common application problems encountered is the failure to provide a dc return path for bias current in ac-coupled op amp or in-amp circuits. In Figure 1 a capacitor is connected in series with the noninverting (+) input of an op amp. This ac coupling is an easy way to block dc voltages associated with the input voltage (VIN). This is especially useful in high gain applications, where even a small dc voltage at amplifier input can limit the dynamic range or even result in output saturation. However, capacitive coupling into a high-impedance input without providing a dc path for current flowing in the positive input leads to problems. OP AMP+VSVS0.1F0.1FVINR2VOUTR307034-001Figure 1. A Nonfunctional AC-Coupled Op Amp Circuit The input bias current flows through the coupling capacitor, charging it, until the common-mode voltage rating of the amplifiers input circuit is exceeded or the output is driven into limits. Depending on the polarity of the input bias current, the capacitor charges up toward the positive supply voltage or down toward the negative supply. The bias voltage is amplified by the closed-loop dc gain of the amplifier. This process can be lengthy. For example, an amplifier with a field effect transistor (FET) input, having a 1 pA bias current, coupled via a 0.1-F capacitor, has an IC charging rate, I/C, of 1012/107= 10 V per sec or 600 V per minute. If the gain is 100, the output drifts at 0.06 V per minute. Therefore, a casual lab test, using an ac-coupled scope, may not detect this problem, and the circuit may not fail until hours later. It is important to avoid this problem altogether. OP AMP+VSVS0.1F0.1FVINR2R1C1VOUTR3DESIGN EQUATIONS3dB INPUT BW = 1/(2R1 C1)R1 IS TYPICALLY SET EQUAL TOTHE PARALLEL COMBINATIONOF R2 AND R3.07034-002Figure 2. Correct Method for AC- Coupling an Op Amp Input for Dual-Supply Operation Figure 2 shows a simple solution to this common problem. In this example, a resistor is connected between the op amp input and ground to provide a path for the input bias current. To minimize offset voltages caused by input bias currents, which track one another when using bipolar op amps, R1 is usually set...
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This note was uploaded on 01/17/2011 for the course EEE 5320 taught by Professor Dr.robertfox during the Fall '10 term at University of Florida.
- Fall '10