Differential and Operational Amplifiers PDF

IT2016 Differential and Operational Amplifiers Differential Amplifier The main feature of this amplifier is the very large gain when opposite signals are applied to the inputs. o In...

IT2016 Differential and Operational Amplifiers Differential Amplifier The main feature of this amplifier is the very large gain when opposite signals are applied to the inputs. o In single-ended operation, a single input signal applies and operates both transistors, resulting in output from both collectors. o In double-ended operation, two input signals are applied, resulting in the difference of the signals applied to both inputs. o In common-mode operation, the common input signal results in opposite signals at each collector, these signals canceling, so that the resulting output signal is zero. DC Biasing Consider the DC bias operation of the circuit with ac inputs obtained from voltage sources, the dc voltage at each input is essentially connected to 0𝑣. With each base voltage at 0𝑣, the common- emitter dc bias voltage is 𝑉𝐸 = 0𝑣 − 𝑉𝐵𝐸 = −0.7𝑣 The emitter dc bias current is then 𝑉𝐸 − (−𝑉𝐸𝐸 ) 𝑉𝐸𝐸 − 0.7𝑣 𝐼𝐸 = ≈ 𝑅𝐸 𝑅𝐸 Assuming that the transistors are well-matched (as would occur in an IC unit), we obtain 𝐼𝐸 𝐼𝐶1 = 𝐼𝐶2 = 2 resulting in a collector voltage of 𝐼𝐸 𝑉𝐶1 = 𝑉𝐶2 = 𝑉𝐶𝐶 − 𝐼𝐶 𝑅𝐶 = 𝑉𝐶𝐶 − 𝑅𝐶 2 Example: Calculate the DC voltages and currents in the Solution: circuit. Find 𝐼𝐸 , 𝐼𝐶 , and 𝑉𝐶. 𝑉𝐸𝐸 − 0.7𝑣 9𝑣 − 0.7𝑣 𝐼𝐸 = = ≈ 2.5 𝑚𝐴 𝑅𝐸 3.3 𝑘Ω The collector current is then 𝐼𝐸 2.5 𝑚𝐴 𝐼𝐶 = = = 1.25 𝑚𝐴 2 2 resulting in a collector voltage of 𝑉𝐶 = 𝑉𝐶𝐶 − 𝐼𝐶 𝑅𝐶 = 9𝑣 − (1.25 𝑚𝐴)(3.9 𝑘Ω) ≈ 4.1𝑣 Note: The common-emitter voltage is thus −0.7𝑣, whereas the collector bias voltage is near 4.1𝑣 for both outputs. 01 Handout 1 *Property of STI  [email protected] Page 1 of 4  IT2016 Operational Amplifiers Op-Amp is a very high gain differential amplifier with high input impedance and low output impedance. It is a low-cost integrating circuit consisting of transistors, resistors, and capacitors. Typical uses of these are to provide voltage amplitude changes (amplitude and polarity), oscillators, filter circuits, and many types of instrumentation circuits. Op-Amp Characteristics Terminals The positive power supply terminal. The negative power supply terminal. The inverting input is labeled−on the schematic symbol. The non-inverting input is labeled + on the schematic symbol. The fifth terminal, of course, is the output. Properties Infinite open-loop gain. It is the gain without feedback, and this is equal to differential gain, which is also called zero common-mode gain. Infinite Input impedance. Op-amp has input current, which is approximately equal to zero, 𝑖𝑖 ≈ 0𝐴, thus, the input impedance is very high in high-grade op-amp and small 𝑚𝐴 input current in low-grade op-amp. Zero Output Impedance. Op-amps act as a perfect internal voltage source, which means that there is no internal resistance. The output impedance is in series with the load thus reducing its output voltage. Voltage Transfer Characteristics The graph that relates the output voltage to the input voltage is called the voltage transfer curve and is fundamental in designing and understanding amplifier circuits. Note the two distinct regions of operation: one around 𝑉𝑖 = 0𝑣, the linear region where the output changes linearly with respect to input, and the other at which changes in 𝑉𝑖 has little effect on 𝑉𝑂 , the saturation region (non-linear behavior). Circuits with operational amplifiers can be designed to operate in both regions. In the linear region, the slope of the line relating 𝑉𝑂 to 𝑉𝑖 is very large; indeed, it is equal to the open- loop gain 𝐴. Ideal Op-Amp Characteristics Negative Feedback o Feedback is when you connect the output of a circuit back to its input so that the output “feeds back” into the circuit. When an increase in the output is fed back, it causes a decrease in the output. Negative feedback is like self-correction. Infinite Gain and Zero Input Current. o When an op-amp is used in negative feedback, the consequence of infinite gain is that the two inputs will be forced to be equal. That is, the op-amp will output whatever voltage is necessary to make the two inputs equal. 01 Handout 1 *Property of STI  [email protected] Page 2 of 4  IT2016 Output Saturation o Naturally, an op-amp can only output voltages contained within the range of its power supply. o When the output voltage implied by the circuit would exceed the possible range, the op-amp is said to saturate, and it just outputs its maximum or minimum possible voltage instead. o When op-amp output saturation causes the signal to be cut off close to the rails, we say that the signal is clipped. Zero output voltage if the input voltage is zero. o It must follow Ohm’s Law. Basic Input/Output Stages An op-amp contains several differential amplifier stages to achieve a very high voltage gain. Single-Ended Input - This operation results when the input signal is connected to one input with the other input connected to the ground. The input is applied to the plus input (with minus It shows an input signal applied to the minus input, input at ground), which results in an output having the output then being opposite in phase to the the same polarity as the applied input signal. applied signal. Double-Ended (Differential) Input - In addition to using only one input, it is possible to apply signals at each input—this being a double-ended operation. It shows an input, 𝑉𝑑 , applied between the two input terminals (recall that neither input is at ground). Thus, the amplified output in-phase is applied between the It shows the same action resulting when two plus and minus inputs. separate signals are applied to the inputs, the difference signal being 𝑉𝑖1 − 𝑉𝑖2. Double-Ended Output - An input applied to either input will result in outputs from both output terminals; these outputs always being opposite in polarity. 01 Handout 1 *Property of STI  [email protected] Page 3 of 4  IT2016 Op-Amp Common-Mode Operation When the same input signals are applied to both inputs, common-mode operation is performed. Ideally, the two inputs are equally amplified, and since they result in opposite-polarity signals at the output, these signals cancel, resulting in 0𝑣 output. Since there are noise (any unwanted input signal) common to both inputs, the differential connection tends to provide attenuation of this unwanted input, while providing an amplified output of the difference signal. This operating feature is called common-mode rejection. References: Boylestad, R. & Nashelsky, R. (2013). Electronic devices and circuit theory (11th ed.). Pearson. Fernandez-Canque, H. (2017). Analog electronics applications: Fundamentals of design and analysis. CRC Press. Schuler, C. (2019). Electronics: Principles and Applications (9th ed.). McGraw-Hill. Stephan, K. (2015). Analog and mixed-signal electronics. Wiley. Storey, N. (2017). Electronics. A systems approach (6th ed.). Pearson. 01 Handout 1 *Property of STI  [email protected] Page 4 of 4