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ECE, ME – I SEM BASIC ELECTRONICS OPERATIONAL AMPLIFIER - + Contents A)Basics of Op-amp B)Ideal Characteristics c)Applilcations Operational Amplifier It refers to a circuit that is composed of transistors, resistors, and capacitors, and has a very large Thévenin input resistance and very small Thévenin output resistance. It behaves, over at least a portion of its operating range, as a high-gain linear voltage amplifier. It is an integrated circuit component, and has two signal input terminals and one signal output terminal. As an integrated circuit, the op-amps needs DC power supplies for its operating. Using the op-amps we can construct many useful complex circuits. Therefore the op-amps are the versatile components for constructing circuits Typical Op Amp IC Packages Op Amp Pin-out connections Dual-in-Line Plastic Package Top view Op Amp Pin-outs Metal Can Two matched transistors form a differential amplifier Transisterized Differential Amplifier Double ended input - double ended output diff. amp: vout=A (v1-v2) vout v1 - + v2 Op Amp stages with pin-outs of IC741 7 2 6 3 4 Ideal Operational Amplifier An ideal Operational Amplifier is basically a 3-terminal device that consists of two high impedance inputs, one an Inverting input marked with a negative sign, ("-") and the other a Non-inverting input marked with a positive plus sign ("+"). The amplified output signal of an Operational Amplifier is the difference between the two signals being applied to the two inputs. In other words the output signal is a differential signal between the two inputs and the input stage of an Operational Amplifier is in fact a differential amplifier as shown below. Equivalent Circuit for Ideal Operational Amplifiers The transfer characteristic of an op- amp is shown: Explanation It includes Three regions 1. Negative saturation region, the output voltage is fixed to - Vcc (<0): 2. Positive saturation region, the output voltage is fixed to +Vcc (>0) 3. Linear region, the output voltage is determined linearly by input difference. where, A is called the open-loop voltage gain of an op-amp, which is usually excess of 10,000. IDEALIZED CHARACTERISTIC a) Voltage Gain, (A)Infinite - The main function of an operational amplifier is to amplify the input signal and the more open loop gain it has the better, so for an ideal amplifier the gain will be infinite. b) Input impedance,(Zin)Infinite - Input impedance is assumed to be infinite to prevent any current flowing from the source supply into the amplifiers input circuitry. c) Output impedance, (Zout)Zero - The output impedance of the ideal operational amplifier is assumed to be zero so that it can supply as much current as necessary to the load. d) Bandwidth, (BW)Infinite - An ideal operational amplifier has an infinite Frequency Response and can amplify any frequency signal so it is assumed to have an infinite bandwidth. e) Offset Voltage, (Vio)Zero - The amplifiers output will be zero when the voltage difference between the inverting and non- inverting inputs is zero Points to Remember However, real Operational Amplifiers such as the commonly available uA741, for example do not have infinite gain or bandwidth but have a typical "Open Loop Gain" which is defined as the amplifiers output amplification without any external feedback signals connected to it and for a typical operational amplifier is about 100dB at DC (zero Hz). This output gain decreases linearly with frequency down to "Unity Gain" or 1, at about 1MHz and this is shown in the following open loop gain response curve. Op amp Characteristics Common Mode Rejection Ratio (CMRR) It is the ability of an op amp to reject the signal which is present at its both inputs simultaneously i.e. the common mode signal CMRR = Ad / ACM, where ACM is common mode voltage gain defined by Vout / VCM, Ad is differential gain of Op-Amp. Ideally CMRR is infinite For IC 741 it is 90 dB Opamp Slew Rate •Effect of slew-rate limits output for sinusoidal waveforms. •Slew rate is caused by the finite response time of the circuit elements of an op amp i m ax It limits the highest possible frequency of operation dvi Vi cost Vi dv0 dt SR dt •Limited BW, m ax 17 Offset Voltage Input Offset Voltage Small voltage needed to be applied between INV and NI terminals to get zero output voltage Ideally it should be 0 V (CM operation) Output Offset Voltage Under common mode operation, output voltage should be zero, but due to mismatch is devices it is non-zero (Can be corrected by applying voltage between Pins 1 and 5 of IC 741) Characteristics (Cont’d) Power-supply rejection ratio. The power-supply rejection ratio PSRR is the ratio of the change in input offset voltage to the corresponding change in one power-supply, with all remaining power voltages held constant. The PSRR is also called "power supply insensitivity". Typical values are in:V / V or mV/V Input Bias Current: The average of the currents into the two input terminals with the output at zero volts. Input Offset Current : The difference between the currents into the two input terminals with the output held at zero Open Loop Gain AOL is ideally infinite In case of typical practical op amp like IC 741 it is of the order of 105 i.e. about 100 dB Input Resistance RI is ideally infinite In case of IC 741 it is 2 MΩ Output Resistance RO is ideally zero In case of IC 741 it is 75 Ω Open-loop Frequency Response Curve Product of the gain against frequency is constant at any point along the curve. Also that the unity gain (0dB) frequency also determines the gain of the amplifier at any point along the curve. This constant is generally known as the Gain Bandwidth Product or GBP. GBP = Gain x Bandwidth or A x BW. Example For example, from the graph above the gain of the amplifier at 100kHz = 20dB or 10, then the GBP = 100,000Hz x 10 = 1,000,000. Similarly, a gain at 1kHz = 60dB or 1000, therefore the GBP = 1,000 x 1,000 = 1,000,000. The same!. An Operational Amplifiers Bandwidth The operational amplifiers bandwidth is the frequency range over which the voltage gain of the amplifier is above 70.7% or -3dB (where 0dB is the maximum) of its maximum output value as shown below. Here we have used the 40dB line as an example. The -3dB or 70.7% of Vmax down point from the frequency response curve is given as 37dB. Taking a line across until it intersects with the main GBP curve gives us a frequency point just above the 10kHz line at about 12 to 15kHz. We can now calculate this more accurately as we already know the GBP of the amplifier, in this particular case 1MHz Key Points Open loop gain of op amp is defined as: AOL = Vo / VD where VD = VNI – VINV Open loop gain of op amp is very high (ideally infinite). Any small difference between VNI and VINV results into saturation of output voltage ±VSAT i.e. for VNI < VINV output is – VSAT and for VNI > VINV output is + VSAT Value of VSAT is limited by the supply voltage of op amp Important features of Op Amp 1. High open loop gain (ideally infinite) which implies that even the smallest difference between the two inputs results into saturated output voltage 2. High input impedance (ideally infinite) implies that there is no current flowing into the input of an op amp Virtual Ground a) These two conditions give rise to VIRTUAL GROUND, where the voltages at both the inputs are maintained at exactly same level. b) To achieve this condition, a feedback circuit between the output and the inverting input terminal of the op amp is necessary. c) This results into many applications of op amp, which qualify it to be OPERATIONAL: adder, subtractor, multiplier, divider etc. Opamp can be used in two configuration A) Open Loop configuration: a) There is no connection from output to input. b) When connected in open-loop configuration, the op-amp functions as a high gain amplifier. These are three open loop op-amp configurations. Differential Amplifier Inverting Amplifier Non-Inverting Amplifier Note: These configurations are classed according to the number of inputs used and the terminal to which the input is applied when a single input is used. Disadvantages of open loop configurations The open loop of the op-amp is very high. Therefore only the smaller signals having low frequency may be amplified accurately without distortion. Open loop Voltage gain of the op-amp is not a constant voltage gain varies with changes in temperature and power supply as well as mass production techniques. This makes op-amp unsuitable for many linear applications Bandwidth of most open loop op-amps is negligibly small or almost zero therefore op-amp is impractical in ac applications. Note: For these reasons, the open loop op-amp is generally not used in linear applications. In certain applications, the open loop op-amp is purposely used as a non -linear device; that is a square wave output is obtained by applying a relatively large input signal Closed Loop Configuration There is connection from output to input The use of feedback i.e. an output signal is fed back to the input either directly or via another network. If the signal fed back is of opposite polarity or out of phase of 180 degree. with respect to the input signal, the feedback is called negative feedback. Negative feedback is also known as degenerative feedback. When used in amplifiers, negative feedback stabilizes the gain, increases the bandwidth and changes the input and output resistance. Other benefits include a decrease in harmonic or non-linear distortion and reduction in the effect of input offset voltage at the output. It is also reduces the effect of variations in the temperature and supply voltages on the output of the op-amp. These are three open loop op-amp configurations a) Differential Amplifier b) Inverting Amplifier c) Non-Inverting Amplifier Inverting Amplifier Multiplier/Divider circuit, whose gain is decided by the values of resistors Rin Rf Vin + Vout Inverting Amplifier Analysis Due to virtual ground: Iin= Vin / Rin= If = – ( Vout / Rf ) Vout= – (Rf / Rin ) Vin Rf > Rin →multiplier Rf < Rin→ divider Iin R If Rf in Vin + Vout Non-inverting Amplifier Due to virtual ground: VINV = VNI = Vin If = (Vout–Vin) / Rf = Iin= Vin / Rin Vout = [1+ (Rf / Rin )] Vin + Vin If Vin Vout Iin Rf Rin The Difference Amplifier R Since v-= v+ vo 2 (v v ) R 1 2 1 For R2= R1 vo (v1 v2) This circuit is also called a differential amplifier, since it amplifies the difference between the input signals. Rin2 is series combination of v o v- i R v- i R 2 2 1 2 R1 and R2 because i+ is zero. R R R R For v2=0, Rin1= R1, as the v- 2 ( v v- ) 2 v- 2 v 1 circuit reduces to an R 1 R R 1 inverting amplifier. 1 1 1 R For general case, i1 is a Also, v 2 v function of both v1 and v2. R R 2 1 2 Unity Gain Amplifier (Non-inverting Buffer – Vout=Vin Vin + Summary Op amp can be used for various (mathematical) operations like addition, subtraction, multiplication, division, differentiation, integration etc. Infinite open loop gain and infinite input resistance give rise to VIRTUAL GROUND Op amp is available in a simple to use IC form which require dual supply Many other applications can be implemented using op amp

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