EE105 Fall 2014 Microelectronic Devices and Circuits. Ideal vs Non-ideal Op Amps

Transcription

1 EE05 Fall 204 Microelectronic Devices and Circuits Prof. Ming C. Wu 5 Sutardja Dai Hall (SDH) vs Non-ideal Op Amps Op Amp A 0 Non-ideal Op Amp A < < > 0 Other non-ideal characteristics: Finite common-mode rejection ratio (CMRR) Output voltage is not just proportional to the difference input voltage but also their absolute values Common-mode input resistance dc error sources (offset voltage, input bias current, offset current) Output voltage, slew rate, maximum current limits 2

2 Classic Feedback Systems v d v i v f v o Av d v f βv o v o A v i + Aβ # Aβ & # T & % ( A β$ + Aβ v % ( ' $ + T ' A transfer function of open -loop amplifier or open-loop gain. β transfer function of feedback network. T loop gain Aβ For negative feedback: T(s) > 0 For positive feedback: T(s) < 0 3 Gain Error and Fractional Gain Error Gain Error: GE (ideal gain) - (actual gain) For non-inverting amplifier, GE β $ ' & T ) & β % +T $ & ) & ( β % +T Fractional or percentage error: ' ) ) ( FGE GE # & % ( β$ + T ' β + T T for T >> 4 2

3 Gain Error Example: Noninverting Amplifier Problem: Find actual gain and gain error for an amplifier Given data: closed-loop gain of 200 (46 db), open-loop gain of op amp is 0,000 (80 db). Approach: Amplifier is designed to give ideal gain and deviations from ideal case are determined. Note: and are not normally designed to compensate for finite open-loop gain of amplifier. Analysis: " T % " % $ ' with T Aβ 0 4 $ ' 50 # + T & # 200& β 200 A " 50 % v 200$ ' 96 FGE # + 50& Note : FGE T Gain of Noninverting Amplifier ( ) A( v i βv o ) v o Av id i v v o A v i + Aβ " Aβ % $ ' " T % $ ' β # + Aβ & β # +T & T Aβ is called the loop gain For T >> β + For i 0, v β + + v o βv o Feedback factor v id v i v v i βv o v id v i T +T v i v i +T Although no longer zero, v id is small for large loop gain T. 6 3

4 Output Resistance (Both Noninverting and Inverting Amplifiers) The output terminal is driven by test source and current is calculated to determine output resistance (all independent sources are turned off). Note that the equivalent circuit is same for both inverting and non -inverting amplifiers. ut 7 Output Resistance (cont d) Analysis : i o + i 2 i o Av id and i 2 Also, v id v and v ut + Aβ + ut + T + Thus, shunt feedback reduces ut. Typically, ( ) β + T ( ) << + ( ) and ut + T. For infinite T, ut

5 Nonideal Op Amps Open-loop Gain: Design Example Problem: Design non-inverting amplifier and find the required open-loop gain Given Data: 35 db, ut 0.2 Ω, 250 Ω Analysis: 35dB 20dB β 56.2 ut + Aβ 0.2 Ω A ( + ( * - A 56.2* 250 β) ut, ) x0 4 or 96.9 db, 9 Input Resistance of Noninverting Amplifier Assuming i << i 2 gives i i 2 Test voltage source is applied to input and current is calculated: v v i i 2 v + v o βv o v β Av id ( ) Aβ ( v ) v Aβ + Aβ T +T v R in ( +T ) T +T +T ( ) 0 5

6 Gain of Inverting Amplifier i v v i ( id ) and i 2 v id i 2 i and v o Av id ( ) v o Let i be the current going from the input source through and i 2 be the current going through into the amplifier output. After some algebra, v " o R % 2 Aβ $ ' v i # & + Aβ with β + " T % $ ' # +T & Input Resistance of Inverting Amplifier R in + v + v i i + i 2 v + v v o v + v + Av G i + +T v removed! R R in + 2 $ # & + " + A % + A For large A, R in 2 6

7 Nonideal Amplifier Summary Inverting and Noninverting Amplifier β + and T Aβ 3 Feedback Amplifier Categories Voltage Amplifiers - Series-Shunt Feedback oltage amplifier should have a high input resistance to measure the desired voltage and a low output resistance to drive the external load. To achieve the desired behavior, the input ports of the amplifier and feedback network are connected in series, and the output ports are connected in parallel (shunt). 4 7

8 Feedback Amplifier Categories Transresistance Amplifiers - Shunt-Shunt Feedback A transresistance amplifier converts an input current to an output voltage. Thus it should have a low input resistance to sink the desired current and a low output resistance to drive the external load. To achieve the desired behavior, the input ports of the amplifier and feedback network are connected in parallel (shunt), and the output ports are connected in parallel (shunt). 5 Feedback Amplifier Categories Current Amplifiers - Shunt-Series Feedback A current amplifier should provide a low resistance current sink at the input and a high resistance current source at its output. The input ports of the amplifier and feedback network are connected in parallel, and the output ports are connected in series. 6 8

9 Feedback Amplifier Categories Transconductance Amplifiers - Series-Series Feedback The transconductance amplifier converts an input voltage to an output current. This amplifier should have a high input resistance and a high output resistance. The input ports and the output ports of the amplifier and feedback networks are connected in series. 7 Feedback Amplifier Category Summary 8 9