Op-amp parameters Notes PDF

Input Offset Voltage Ideal op-amp: produces 0-volt output for a 0-volt input. Practical op-amp: a small DC voltage (Vout(error)) appears at the output when no differential input voltage is applied. Cause: Mismatch between the base-emitter voltages of the differential input sta...

Input Offset Voltage Ideal op-amp: produces 0-volt output for a 0-volt input. Practical op-amp: a small DC voltage (Vout(error)) appears at the output when no differential input voltage is applied. Cause: Mismatch between the base-emitter voltages of the differential input stage of the op-amp. 9/19/2021 ECMIXS30 - MALAAY - 2021 26 Input Offset Voltage Input Offset Voltage, Vos The differential DC voltage required between the inputs to force the output 0 volts. Typical values: < 2mV (0V ideal case) Input Offset Voltage Drift Parameter related to Vos that specifies how much change occurs in the Vos for each degree change in temperature. Typical values: 5µV/°C to 50µV/°C Higher nominal value, higher drift. 9/19/2021 ECMIXS30 - MALAAY - 2021 27 Input Bias Current The DC current required by the inputs of the amplifier to properly operate the first stage. The average of both input currents computed as follows: 𝐼1 + 𝐼2 𝐼𝑏𝑖𝑎𝑠 = 2 9/19/2021 ECMIXS30 - MALAAY - 2021 28 Input Impedance Differential Input Impedance (a) The total resistance between the inverting and non-inverting inputs. Measured by determining the change in bias current for a given change in differential input voltage. Common-Mode Input Impedance (b) The resistance between each input and ground. Measured by determining the change in bias current for a given change in common-mode input voltage. 9/19/2021 ECMIXS30 - MALAAY - 2021 29 Input Impedance 9/19/2021 ECMIXS30 - MALAAY - 2021 30 Input Offset Current The difference of the input bias currents expressed as an absolute value. 𝐼𝑜𝑠 = 𝐼1 − 𝐼2 Actual magnitude of offset current are usually an order of magnitude (10x) less than the magnitude current. In many applications, the offset current can be neglected. High gain, high input impedance, low Ios. 9/19/2021 ECMIXS30 - MALAAY - 2021 31 Input Offset Current 9/19/2021 ECMIXS30 - MALAAY - 2021 32 Output Impedance The output impedance of an op-amp can be measured at the output terminal as shown on the figure. 9/19/2021 ECMIXS30 - MALAAY - 2021 33 Slew Rate The maximum rate of change of the output voltage in response to a step input voltage. It is dependent upon the high frequency response of the amplifier stages within the op-amp. Connection: non-inverting, unity gain configuration which yields a worst case (slowest) slew rate. 9/19/2021 ECMIXS30 - MALAAY - 2021 34 Slew Rate 9/19/2021 ECMIXS30 - MALAAY - 2021 35 Slew Rate 9/19/2021 ECMIXS30 - MALAAY - 2021 36 Slew Rate Example: The output voltage of a certain op-amp appears as shown in the figure in response to a step input. Determine the slew rate. 9/19/2021 ECMIXS30 - MALAAY - 2021 37 Slew Rate Given: +Vmax = 9V -Vmax = -9V ∆𝑡 = 1𝜇𝑠 Note: Since this response is not ideal, the limits are taken at the 90% points. 9/19/2021 ECMIXS30 - MALAAY - 2021 38 Slew Rate Given: +Vmax = 9V -Vmax = -9V ∆𝑡 = 1𝜇𝑠 ∆𝑉 +9 −(−9) 𝑽 𝑆𝑅 = = = 𝟏𝟖 ∆𝑡 1𝑥10−6 𝝁𝒔 9/19/2021 ECMIXS30 - MALAAY - 2021 39 Frequency Response The internal amplifier stages that make up an op-amp have voltage gains limited by junction capacitances. Op-amps has no internal coupling capacitor, but the low frequency response extends down to DC (0Hz). 9/19/2021 ECMIXS30 - MALAAY - 2021 40 Comparison of Op-Amp Parameters 9/19/2021 ECMIXS30 - MALAAY - 2021 41 Op-amps with negative feedback Lesson 1.3 9/19/2021 ECMIXS30 - MALAAY - 2021 42 Negative Feedback One of the most useful concepts in Electronics, particularly in op-amp applications. The process whereby a portion of the output voltage of an amplifier is returned to the input with a phase angle that opposes the input signal. 9/19/2021 ECMIXS30 - MALAAY - 2021 43 Negative Feedback The inherent open-loop voltage gain of a typical op-amp is very high (>100,000). An extremely small input drives the op-amp into its saturated output states. Even the input offset voltage of the op-amp can drive it into saturation. Example: Vin = 1mV and Aol = 100,000 Vin Aol = (1mV)(100,000) = 100V 9/19/2021 ECMIXS30 - MALAAY - 2021 44 Negative Feedback 9/19/2021 ECMIXS30 - MALAAY - 2021 45 Negative Feedback Op-amp without negative feedback: Severely restricted and limited to comparators and other applications. Op-amp with negative feedback: the closed-loop gain (Acl) can be reduced and controlled so that the op-amp can function as a liniear amplifier. Negative feedback provides control for the input and output impedances and amplifier bandwidth providing a controlled and stable voltage gain. 9/19/2021 ECMIXS30 - MALAAY - 2021 46 Negative Feedback 9/19/2021 ECMIXS30 - MALAAY - 2021 47 Closed-loop Gain The voltage gain of an op-amp with external feedback. The amplifier configuration consists of the op-amp and an external negative feedback circuit that connects the output to the inverting input. Determined by the external component values and can be precisely controlled by them. 9/19/2021 ECMIXS30 - MALAAY - 2021 48 Non-inverting Amplifier An op-amp connected in a closed-loop configuration as a non- inverting amplifier with a controlled amount of voltage gain. The input signal is applied to the non-inverting input (+) then the output is applied back to the inverting input (-) through the feedback circuit (closed loop) formed by an input resistor Ri and a feedback resistor Rf which creates a negative feedback. 9/19/2021 ECMIXS30 - MALAAY - 2021 49 Non-inverting Amplifier The resistors Ri and Rf form a voltage-divider circuit which reduces the output voltage Vout and connects the reduced voltage Vf to the inverting input. The feedback voltage is expressed as: 𝑹𝒊 𝑽𝒇 = 𝑽𝒐𝒖𝒕 𝑹𝒊 + 𝑹𝒇 9/19/2021 ECMIXS30 - MALAAY - 2021 50 Non-inverting Amplifier The difference of input voltage Vi and the feedback voltage Vf is the differential input to the op- amp. This differential voltage is amplified by the open-loop voltage gain of the op-amp (Aol) and produces the output voltage expressed as: 𝑽𝒐𝒖𝒕 = 𝑨𝒐𝒍 (𝑽𝒊𝒏 − 𝑽𝒇 ) 9/19/2021 ECMIXS30 - MALAAY - 2021 51 Non-inverting Amplifier Attenuation of the feedback circuit 𝑹𝒊 𝑩= 𝑹𝒊 + 𝑹𝒇 Closed-loop gain of a non-inverting amplifier 𝑹𝒇 𝑨𝒄𝒍(𝑵𝑰) = 𝟏 + 𝑹𝒊 9/19/2021 ECMIXS30 - MALAAY - 2021 52 Example Problem: Determine the gain of the amplifier shown in the figure. The open-loop voltage gain of the op-amp is 100,000. Solution: Non-inverting op-amp configuration 𝑅𝑓 𝐴𝑐𝑙(𝑁𝐼) = 1 + 𝑅𝑖 100𝑘 𝐴𝑐𝑙(𝑁𝐼) = 1 + = 𝟐𝟐. 𝟑 4.7𝑘 9/19/2021 ECMIXS30 - MALAAY - 2021 53 Voltage-follower A special case configuration of a non-inverting amplifier where all of the output voltage is fed back to the inverting input by a straight connection. The straight feedback connection has a voltage gain of 1 (no gain). 𝐴𝑐𝑙(𝑉𝐹) = 1 9/19/2021 ECMIXS30 - MALAAY - 2021 54 Voltage-follower Important features: very high input impedance, very low output impedance These features make it a nearly ideal buffer amplifier for interfacing high impedance sources and low impedance loads. 9/19/2021 ECMIXS30 - MALAAY - 2021 55 Inverting Amplifier The input signal is applied through a series of input resistor Ri to the inverting input (-). The output is fed back through Rf to the same input. The non-inverting input (+) is grounded. 9/19/2021 ECMIXS30 - MALAAY - 2021 56 Inverting Amplifier 9/19/2021 ECMIXS30 - MALAAY - 2021 57 Inverting Amplifier Input current 𝑰𝒊𝒏 = 𝑰𝒇 Overall gain of the inverting amplifier 𝑹𝒇 𝑨𝒄𝒍(𝑰) =− 𝑹𝒊 Important note: The negative sign only indicates inversion! 9/19/2021 ECMIXS30 - MALAAY - 2021 58 Example Problem: Given the op-amp configuration in the figure shown, determine the value of Rf require to produce a closed-loop voltage gain of -100. Solution: 𝑅𝑓 𝐴𝑐𝑙(𝐼) = 𝑅𝑖 𝑅𝑓 −100 = 2.2𝑘 𝑅𝑓 = 𝟐𝟐𝟎𝒌𝜴 9/19/2021 ECMIXS30 - MALAAY - 2021 59