Brief Summary
This lecture provides a comprehensive review of operational amplifiers (op-amps), covering ideal versus real characteristics, internal stages, key parameters, and datasheet specifications. It emphasizes the differences between ideal and practical op-amps, discusses the impact of parameters like input bias current and offset voltage, and explains the significance of thermal drift and common-mode rejection ratio (CMRR). The lecture also touches on transient response, slew rate, bandwidth, and supply current, highlighting their importance in op-amp applications.
- Ideal vs. Real Op-Amps
- Key Parameters and Characteristics
- Datasheet Specifications
- Thermal Drift and Compensation
- Applications and Future Modules
Ideal vs. Real Op-Amps and Golden Rules
The lecture begins by revisiting the characteristics of an ideal op-amp, including infinite voltage gain, infinite input impedance, and zero output impedance. It also restates the two golden rules: no current flows into the op-amp's inputs, and the output adjusts to minimize the voltage difference between the inputs. The discussion then transitions to the differences between ideal and real (practical) op-amps, noting that real op-amps have finite voltage gain (around 10^5 to 10^9), a finite gain bandwidth product (1 to 20 MHz), input resistance (10^6 to 10^12 ohms), and output resistance (100 to 1000 ohms).
Power Supplies, Output Configurations, and Internal Op-Amp Formula
The lecture briefly touches on balanced and unbalanced power supplies, as well as single-ended and double-ended outputs. It explains that balanced power supplies are more commonly used, while single-ended outputs are with respect to ground, and double-ended outputs are floating. The internal op-amp formula, Vout = Gain * (V+ - V-), is reviewed, emphasizing that the output polarity depends on the relative signal levels at the inverting and non-inverting terminals.
Feedback Types and Internal Stages of Op-Amps
The discussion covers feedback, distinguishing between negative feedback (used for amplifiers) and positive feedback (used for oscillators). The internal stages of an operational amplifier are outlined, including the differential amplifier, intermediate stage, level shifter stage, and output stage. It is noted that MOSFETs are now commonly used in place of BJTs in these internal circuits.
Key Parameters of Operational Amplifiers
The lecture details several key parameters of operational amplifiers. Open-loop gain is defined as the voltage gain without feedback, typically in the thousands. Input impedance is finite but generally greater than 1 megaohm, and can be several hundred megaohms for FET inputs. Output impedance is around 1 to 2 ohms. The bandwidth of a practical op-amp in open-loop is small, but negative feedback can increase it. Input offset voltage is the small voltage required at one of the input terminals to make the output voltage zero when both inputs are grounded.
Input Bias Current and Input Offset Current
The lecture explains input bias current, noting that while ideal op-amps have no current flowing into the input terminals, practical op-amps have small input currents (10^-6 to 10^-14 amperes). These currents are due to the base currents of the input transistors in the differential amplifier. Input bias current is measured as the average of the magnitudes of the two input base currents (IB1 and IB2). Input offset current is defined as the difference in the magnitudes of the input bias currents (IB1 - IB2). It's emphasized that both input bias and offset currents are temperature-dependent.
Virtual Ground and Differential Mode Gain
The concept of virtual ground is explained, where the differential voltage (VD) between the non-inverting and inverting terminals is essentially zero. This occurs because the large open-loop gain of the op-amp minimizes the voltage difference at the input terminals. Even if one terminal is grounded, the other is considered a virtual ground. Differential mode gain (Ad) is the gain with which the differential amplifier amplifies the difference between two input voltages (V1 - V2).
Common-Mode Gain and Common-Mode Rejection Ratio (CMRR)
Common-mode gain refers to how a common-mode input voltage is amplified by the op-amp. If two equal input voltages are applied to the differential amplifier, the output should ideally be zero. However, practical op-amps also depend on the average common level of the two inputs, known as the common-mode signal (VCM), calculated as (V1 + V2) / 2. The output voltage depends on both differential mode gain and common-mode gain, expressed as VO = Ad * VD + ACM * VCM. Common-mode rejection ratio (CMRR) is the ratio of differential gain to common-mode gain (Ad / ACM). Ideally, CMRR should be infinite, but in practice, it is very large and often expressed in decibels.
Input Offset Voltage Nulling and Bias Current Compensation
The lecture discusses the technique to null the output voltage by connecting a potentiometer between compensation pins to adjust the output voltage to zero. It also mentions how input bias currents affect amplifier operation and how to compensate for these effects using appropriate resistance.
Thermal Drift and Datasheet Parameters
Thermal drift is introduced as a critical datasheet parameter, describing how op-amp behavior changes with operating temperature. Bias current, offset current, and offset voltage all change with temperature. The lecture provides an example using the LM741A, where the worst-case drift is 15 microvolts per degree Celsius. A sample calculation illustrates how temperature changes can significantly affect the output voltage, emphasizing the importance of temperature compensation circuits.
Input Resistance, Voltage Range, and Large Signal Voltage Gain
The lecture covers additional datasheet parameters, including input resistance, input voltage range, and large signal voltage gain. Input resistance for the LM741 is a minimum of 2 megaohms, while many op-amps have input impedances over 1 gigaohm. The input voltage range specifies the allowable voltage levels at the input pins for proper op-amp function. Large signal voltage gain is the gain of the op-amp at DC or low frequencies, typically around 200,000.
Output Voltage Swing and Short-Circuit Current
Output voltage swing refers to the range the output can swing, often within a few volts of the power rails. Rail-to-rail op-amps can swing the output to within 100 millivolts of the supply rails. Output short-circuit current is the maximum current the output can source or sink, typically around 25 milliamperes.
Common Mode Rejection Ratio (CMRR) and Power Supply Rejection Ratio (PSRR)
The lecture revisits common mode rejection ratio (CMRR), emphasizing its importance for amplifying circuits. For the LM741, the worst-case CMRR is 70 dB, and typically around 90 dB. Power supply voltage rejection ratio (PSRR) is introduced as a measure of how well the op-amp filters out noise from the power pins. An example illustrates how a 12-volt supply with 100 millivolts of ripple at 120 Hertz can affect the op-amp circuit, highlighting the need for power supply filtering to achieve a good PSRR.
Transient Response, Slew Rate, and Bandwidth
Transient response indicates how quickly the op-amp responds to a pulse input. Slew rate is defined as how fast the output can change with respect to the input signal, providing insight into the maximum frequency and amplitude signal the op-amp can handle without distortion. An example using the LM741 demonstrates how to avoid distortion by lowering either the voltage or the frequency. Bandwidth or gain-bandwidth product describes the gain as a function of frequency for smaller signals.
Supply Current and Module Summary
Supply current is the current drawn from the power supply when there is no load on the op-amp. Low-power op-amps are available that run on less than 10 microamperes. The lecture concludes with a summary of the topics covered, including ICs, substrates, op-amp characteristics, realistic parameters, and datasheet specifications. The next module will cover applications of op-amps and circuit implementation using simulation and hands-on experiments.
