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EE101-Lecture 9 Operational Amplifier Non-inverting amplifier & Inverting amplifier EE101 Fall 2012 Lect 9- Kang 1 EE101 Fall 2012 Lect 9- Kang 2 LM741 Op Amp There are 20 BJTs, 11 Rs and 1 C. EE101 Fall 2012 Lect 9- Kang 3 , whereπ£1 = 0 π£π π 1 π£ = - π π π EE101 Fall 2012 Lect 2- Kang 4 Harold Stephen Black (1898-1983) Inventor of Negative Feedback In 1927 after joining Bell Labs in 1925; published his paper βStablized feedback amplifiersβ In 1934. EE101 Fall 2012 Lect 9- Kang 5 Negative Feedback π π π£π π 1 π£π π£π = π΄(π£π +Ξ² π£π ) π΄ π£π =1βΞ²π΄ π£π A= open loop gain Ξ = feedback gain EE101 Fall 2012 Lect 9- Kang 6 LM741 Op Amp EE101 Fall 2012 Lect 9- Kang 7 Approximate Analysis of the LM 741 Op Amp Circuit KCL at input node π£π +π£π π 1 π£π = - + π π π 1 π£π +π£π π π =0 π£π - (1+ π π π 1 π£π = π΄π π£π or π£π β yields ) π£π (1) π£π π΄π (2) (assumed that π π β 0, ππππππππππ¦ π ππππ) From (1) and (2) π π π£π = (β π 1 π π ) π£π 1β(β π )( 1 For π 1 =10K, π π =20K, π΄π =100,000, 1 π΄π ) π΄ 1βΞ²π΄ = = EE101 Fall 2012 Lect 9- Kang π΄ π£ 1βΞ²π΄ π β2 2 1+100,000 β-2. 8 βπ£π π 1 = π£π βπ£π π π EE101 Fall 2012 Lect 9- Kang π£π π£π πΉπ = 1+ πΉ π 9 EE101 Fall 2012 Lect 9- Kang 10 KCL at node a dictates that 2 1 π£π + = 5πΎ 2.5πΎ 10πΎ 4 + 4= - π£π , thus π£π =-8 V 1 1 ππ = 8 (10πΎ + 2πΎ ) = β4.8ππ΄ EE101 Fall 2012 Lect 2- Kang 11 EE101 Fall 2012 Lect 2- Kang 12 EE101 Fall 2012 Lect 2- Kang 13 EE101 Fall 2012 Lect 2- Kang 14 h EE101 Fall 2012 Lect 2- Kang 15 Common Mode Rejection Ratio The common-mode rejection ratio (CMRR) of a differential amplifier (or other device) is the tendency of the devices to reject the input signals common to both input leads. A high CMRR is important in applications where the signal of interest is represented by a small voltage fluctuation superimposed on a (possibly large) voltage offset, or when relevant information is contained in the voltage difference between two signals. (An example is audio transmission over balanced lines.) Ideally, a differential amplifier takes the voltages, π+ and πβ , on its two inputs and produces an output voltage ππ = π΄π (π+ β πβ ) , where π΄π is the differential gain. However, the output of a real differential amplifier is better described as 1 ππ = π΄π (π+ β πβ ) + 2 π΄ππ (π+ + πβ ), where π΄ππ is the common-mode gain, which is typically much smaller than the differential gain π΄π . The CMRR is defined as the ratio of the powers of the differential gain over the common-mode gain, measured in positive decibels (thus using the 20 log rule): π΄ π΄ CMRR = 10 πππ10 ( π΄ π )2 = 20 πππ10 |π΄ π | dB ππ ππ
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