Chapter 2 MICROELECTRONIC CIRCUITS 37 DIFFERENTIAL AMPLIFIERS Differential amplifiers are high-gain, differential input, and single ended output electronic component sensitive only to the voltage difference at their inputs. ()0 VN IVAVV=− [0-1] The input and output voltages, NV, IV and 0V are referenced to ground. The output 0Vis in phase with the input NVand out of phase by 180degrees. For the very this reason, the input terminal designated by pin 2 is called inverting terminal and distinguished by “−” sign, and the input terminal designated by pin 3 is called non-inverting terminal and designated by “+” sign. The differential amplifiers respond only to the difference signal NIVV− and ignore any signal common to both inputs. This rejection of common signal is called common-mode rejection. Differential amplifiers are used for manipulation of analog signals and to do analog to digital or digital to analog conversion. A differential amplifier operates either in linear region as an operational amplifier (Op-Amp) or in nonlinear region a comparator (one-bit A/D converters). +−EEV−NVIVoutV74326CCV+ Figure 2-41: Differential amplifier38 OPERATIONAL AMPLIFIERS Chapter 2 +−NVIVoutVVAinVoutR Figure 2-42: Circuit model of differential amplifier in linear region of operation. OPERATIONAL AMPLIFIERS Operational amplifiers, op amps, are source-dependent, active circuit elements that deliver power to load while drawing negligible power from their controlling inputs. Op amps are connected to power supplies from which they obtain the energy that they deliver to load. Op amps are versatile high gain, popular analog integrated circuits; serve as a building block of many electronic circuits. The operational amplifier as shown in figure 3.1, consist of three basic stages: 1) An input stage to provide a high resistance with certain amount of voltage gain. 2) A middle stage to provide a high voltage gain. 3) An output stage to provide a low output resistance. The741Aµtype of general-purpose op amp consists of 24 transistors. The name operational amplifier originates from the early use of this type of the amplifiers to perform specific electronic circuit function or mathematical operation. The symbol for an op amp is shown in Figure 3.2. An op amp has at least five terminals. Terminal 6 is for the output voltage. Terminal 2 is called the inverting input, because the output that result from input at this terminal will be inverted. 0 VIVAV=− [0-2]Chapter 2 MICROELECTRONIC CIRCUITS 39 +−CCV+EEV−NVIV0V74326 Figure 2-43: Differential amplifier Terminal 3 is called non-inverting terminal, because the output that result from input at this terminal will have the same polarity. 0 NVAV= [0-3] ()0 NIVAVV=− [0-4] NIVV− is called differential input voltage. Ais open loop gain or differential gain. LF 411CNNSC 8926≈ Figure 2-45: Differential amplifier Terminal 7 is for the positive DC power supply, denoted byCCV+. Terminal 4 is for the negative DC power supply, denoted by EEV−40 OPERATIONAL AMPLIFIERS Chapter 2 Terminal 6 is for the output voltage. The op amp equivalent circuit is shown in Figure 3.2. +−CCV12345678CCVoutputoffset nullinverting inputnon inverting input−offset nullEEV−no connectionpositive power supplyEEV−negative power supply Figure 2-46: op-ampChapter 2 MICROELECTRONIC CIRCUITS 41 offset nulloffset null1Q2Q3Q4Q7Q5Q6Q8Q9Q10Q11Q12Q13Q16Q17Q23Q24Q14Q15Q16Q19Q21Q20Q23Q1C1R2R3R4R9R8R10R11R6R7R5RV+V− Figure 2-47: Schematic of the 741 op-amp 2inRM=Ωfor BJT input stage. 1210inR =Ωfor FET input stage. Input current is very small innA. Output resistance 10 100outR=−Ω is Thevenin’s equivalent resistance Open-loop differential voltage gain 46010 10A =−is the differential voltage gain of the amplifier with no external component. dVVµ=since 0Ais high. The transfer characteristicoutdVVis shown in Figure 3.3. The output voltage is limited to approximately 1 volt below the positive and negative supplies. EE L out CC LVVV VV−<<− [0-5] Error! Objects cannot be created from editing field codes.42 OPERATIONAL AMPLIFIERS Chapter 2 Two regions of operation: 1- linear region 0out dVAV=− is operational amplifier. 2- Saturation region EE L out CC LVVV VV−<<−is comparator (one-bit A/D converter) OPERATIONAL AMPLIFIER SPICE MODELS The student version of PSpice has a library called NOM.LIB, which contains models of three common type of op amp741µ,411LF , and 324LM. Spice parameters for741µmodel With 15ccVV=+and 15EEVV=− ()max 14outVV=± [0-6] 50210A =× [0-7] 1210inR=Ω [0-8] a BJT input stage 75outR=Ω [0-9] 10bfHz= [0-10] 1bwfMHz= [0-11] Spice parameters for 411LF With 15CCVV=+and 15EEVV=−can produce()max 13.5outVV=± 50210A =× [0-12] a FET input stage 1210inR=Ω 50outR=Ω [0-13]Chapter 2 MICROELECTRONIC CIRCUITS 43 20bfHz= [0-14] 4bwfMHz= [0-15] Spice parameters for 324LM With single input15CCVV=+can produce 20 13.5outmV V V≤≤ 50210A=× [0-16] a BJT input stage 2inRM=Ω 50outR=Ω [0-17] 4bfkHz= [0-18] 1bwfMHz= [0-19] If the spice model of op amp is not available, it is possible to represent the op amp by simple model. Spice model of op amp can be classified into three types: namely, dc linear models, ac linear models, and nonlinear macro-models. NONIDEAL OPERATIONAL AMPLIFIER LIMITATION FINITE OPEN-LOOP GAIN FINITE BANDWIDTH (OPEN-LOOP GAIN VARIATION WITH FREQUENCY) FINITE INPUT RESISTANCE NONZERO OUTPUT RESISTANCE EFFECT OF FINITE OPEN-LOOP GAIN The effect of finite open-loop gain VAof operational amplifier on circuit performance will be illustrated for the basic inverting and non inverting configurations. The observation can be extended to other circuit as well. For ideal op-amp VA=∞44 OPERATIONAL AMPLIFIERS Chapter 2 211outinVRVR=+ [0-20] EFFECT OF FINITE OPEN-LOOP VOLTAGE GAIN The open-loop gain VA− is assumed finite and independent of frequency (VAis constant). The op-amp is assumed otherwise ideal (infinite input resistance, zero output resistance). This situation is applicable to dc and/or low frequency applications where the op-amp gain may be assumed independent of frequency. Treatment is limited to two basic configurations (non-inverting and inverting amplifiers). Conclusions drawn from these may be extended to other configurations as well. Results may be extended to the finite band-width case by making the op-amp loop gain a function of frequency. NON INVERTING