6.012 - Microelectronic Devices and Circuits Lecture 22 - Diff-Amp Anal. III: Cascode, µA-741 - Outline • Announcements DP: Discussion of Q13, Q13' impact. Gain expressions. • Review - Output Stages DC Offset of an OpAmp Push-pull/totem pole output stages • Specialty Stages, cont. - more useful transistor pairings The Marvelous Cascode Darlington Connection • A Commercial Op-Amp Example - the µA-741 The schematic and chip layout Understanding the circuit • Bounding mid-band - starting high frequency issues Review of Mid-band concept The Method of Open-Circuit Time Constants Clif Fonstad, 12/1/09 Lecture 22 - Slide 1DC off-set at the output of an Operational Amplifier: DC off-set: The node between Q12 and Q13 is a high impedance node whose quiescent voltage can only be determined by invoking symmetry.* Q16Q19Q18Q20Q21vOUT+-B+ 1.5 V- 1.5 VAQ17Q15Q11Q12Q13Q14Q13'The voltage on these two nodes is equal if there is no input, i.e. vIN1 = vIN2 = 0, and if the circuit is truly symmetrical/matched. This is the high impedance node. Real-world asymmetries mean the voltage on this node is unpredictable. 0 V ≈ 0.6 V + -≈ 0.6 V + -≈ 0 V + + --≈ 0.5 V ≈ 0.6 V ≈ - 0.4 V ≈ - 0.4 V The voltage symmetry says will be at this node. The voltage we need at this node to make VOUT = 0. ≈ 0.6 V + -≈ 0.6 V + -In any practical Op Amp, a very small differential input, vIN1-vIN2, is require to make the voltage on this node (and VOUT) zero. Clif Fonstad, 12/1/09 Lecture 22 - Slide 2DC off-set at the output of an Op Amp, cont: DC off-set: The transfer characteristic, vOUT vs (vIN1 - vIN2), will not in general go through the origin, i.e., vOUT = Avd(vIN1 - vIN2) + VOFFSET In the example in the figure Avd is -2 x 106, and VOFFSET is 0.1 V. VIN2 - VIN1VOUT1V0.5µV-Avd = 2x106VIN2 - VIN10.1V-50nVVOUT+-50 !R+-RvOUTvIN+-AvdInput 1Input 2In a practice, an Op Amp will be used in a feed-back circuit like the example shown to the left, and the value of vOUT with vIN = 0 will be quite small. For this example (in which Avd = -2 x 106, and VOFFSET = In the D.P. you are asked for this value for your design. is only 0.10.1 V) vOUT µV. Clif Fonstad, 12/1/09 Lecture 22 - Slide 3Specialty pairings: Push-pull or Totem Pole Output Pairs A source follower output: - Using a single source follower as the output stage must be biased with a relatively large drain current to achieve a large output voltage swing, which in turn dissipates a lot of quiescent power. Q28+ 1.5 V- 1.5 VIBIASvIN+-vOUT+-vIN goes positivevOUT goes positiveLoad current is supplied through Q28 as it turns on more stronglyRLQ+ 1.5 V- 1.5 VIBIASvIN+-vOUT+-vIN goes negativeNegative vOUT swing limited to -IBIASRL As Q turns off IBIAS flows through load.RLTurns off TheProblem Clif Fonstad, 12/1/09 Lecture 22 - Slide 4Specialty Pairings: The Push-pull or Totem Pole Output A stacked pair of complementary emitter- or source-followers Large input resistance Voltage gain near one Small output resistance Low quiescent power npn or n-MOS follower pnp or p-MOS follower QnQp V+V-vout+-RLvin+VBEn+-+-vin-VEBpQnQpV- V+vout+-RLvin+VGSn+-+-vin-VSGpClif Fonstad, 12/1/09 Lecture 22 - Slide 5Specialty pairings: Push-pull or Totem Pole in Design Prob. Comments/Observations: - The D.P. output stage involves four emitter fol-lower building blocks arranged as two parallel cascades of two emitter follower stages each. - Q20 and Q21 with joined sources at the output node is called a push-pull, or totem pole pair. Q18Q20Q21+ 1.5 V- 1.5 VQ17vOUT+-50!IBIAS2IBIAS3vIN+-- They determine the output resistance of the amplifier. - Ideally the output stage voltage gain is ≈ 1. Clif Fonstad, 12/1/09 Lecture 22 - Slide 6Specialty pairings: Push-pull or Totem Pole in D.P., cont. Inparallel • The input resistance, rout, is highest about zero output, and there current when the input goes negative to pull the output down. Q18Q21+ 1.5 V- 1.5 VvOUT+-50!IBIAS3vIN+-vIN decreaesvOUT decreasesLoad current drawn out through Q21vBE21 increasesvEB21+-Q20+ 1.5 V- 1.5 VQ17vOUT+-50!IBIAS2vIN+-vIN increasesvOUT increasesLoad current supplied through Q20vBE20+-vBE20 increasesrout ≈ rout1|| rout2 rin ≈ rin1|| rin2 Operation: The npn follower supplies current when the input goespositive to push the output up, while the pnp follower sinks it is the output resistance of the two follower stages in parallel. • rin is lowest at this point, too, and is a parallel combination, also. Clif Fonstad, 12/1/09 (discussed in Lecture 21) Lecture 22 - Slide 7! vout= RLIE20evin"vout( )Vt" e" vin"vout( )Vt( )= 2RLIE20sinh vin" vout( )VtSpecialty pairings: Push-pull or Totem Pole, cont. Voltage gain: - The design problem uses a bipolar totem pole. The gain and linearity of this stage depend on the bias level of the totem pole. The gain is higher for with higher bias, but the power dissipation is also. Q20Q21+ 1.5 V- 1.5 Vvout+-50!vin+VBE20+-+-vin-VEB21To calculate the large signal transfer characteristic of the bipolar totem pole we begin with vOUT: ! vOUT= RL"iE 20" iE 21( )The emitter currents depend on (vIN - vOUT): ! iE 20= "IE 20evIN"vOUT( )Vt, iE 21= IE21e" vIN"vOUT( )VtPutting this all together, and using IE21 = - IE20, we have: We can do a spread-sheet solution by picking a set of values for (vIN - vOUT), using the last equation to calculate the vOUT, using this vOUT to calculate vIN, and finally plotting vOUT vs The results are seen on the next slide. vIN. Clif Fonstad, 12/1/09 Lecture 22 - Slide 8Voltage gain, cont.: - With a 50 Ω load and for several different bias levels we find: Clif Fonstad, 12/1/09 Lecture 22 - Slide 9 The gain and linearity are improved by increasing the bias current, but the cost is increased power dissipation. The Av is lowest and rout is highest at the bias point (i.e., VIN = VOUT = 0). rin to the stage is also lowest there.Specialty pairings: Push-pull or Totem Pole in D.P., cont. Reviewing the voltage gain of an emitter follower: r!"ibrorl+-viniin = ib+-vout = AvvinroBias! vout="+1( )ibrl|| ro|| rBias( )vin= ibr#+"+1( )ibrl|| ro|| rBias( )Av=voutvin="+1( )rl|| ro|| rBias( )r#+"+1( )rl|| ro|| rBias( ) $"+1( )rlr#+"+1( )rlIBIASQ25- 1.5 V+ 1.5 Vrtrl+-+-vtvoutNote: - The voltage gains of the third-stage emitter followers (Q25 and Q26) will likelybe very close to one, but that of the stage-four followers might be noticeablyless than one. Clif Fonstad, 12/1/09 Lecture 22 - Slide 10Specialty Pairings: The Cascode Common-source
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