Gain-bandwidth product
Gain bandwidth product is the multiplication of gain at a particular frequency. This is a constant number of an opamp. This number can vary from one opamp to another. Higher the gain bandwidth product number, the higher the speed.
Open loop gain
Open loop gain is the gain by an opamp when it is not connected in any feedback loop. Usually, in an ideal opamp, this number is infinite. In real opamp, it is usually a very high number ranging from 20 dB (10 V/V) to 120 dB (106 V/V). It is usually measured at DC.
Unity gain bandwidth
It is the largest frequency when opamp has some gain (more unity gain). Anything above a gain of 1 V/V is called a gain, so 1 V/V is chosen as a cutoff gain.
Input impedance
For an ideal opamp, the input impedance is considered infinite. This is because we wish the opamp would not load the driving circuit. However, real opamp has finite input impedance. Usually, it is very high in the order of > 106 Ohms.
This number depends mainly on whether the input stage is FET-based or BJT based. If it is FET-based, the input impedance is greater than 1012 Ohms. If it is BJT based input stage, then it could be in the order of 108 Ohms.
Output impedance
Usually, an opamp is considered a voltage source. Its function is to enforce a voltage on a load. So, ideally, the output impedance of an opamp is zero. In reality, every voltage source has some resistance in series, which limits the maximum current that can flow through it.
Using the resistor divider analogy, the source resistance should be zero to enforce the voltage on the load resistor, no matter the value of the resistor. Now in the case of finite output impedance, if the load resistor is equal to the amplifier’s output impedance, the voltage appearing at the load is only half.
Input offset voltage
The input offset voltage is the difference between the inverting and non-inverting terminal of the opamp in the DC state. For an ideal opamp, this offset voltage should be zero. If there is an offset voltage at the input, that offset voltage multiplied by the huge gain of the opamp will saturate the output.
For unity gain opamp, the output is shorted to inverting terminal of the opamp. Usually, it is desired that the output is equal to the input voltage. However, due to offset voltage, the output is a few mV away from input voltage.
The input offset voltage is problematic when very precise measurement is required. When measuring μV of voltage, the gain is set to be huge (>1000V/V). If mV of offset is present, we end up reading the offset of the amplifier rather than the input given to the amplifier. This is because the offset dominates the input signal.
Input bias currents
Input bias currents are a characteristic of operational amplifiers (op-amps) and refer to the small currents that flow into or out of the input terminals of the op-amp.
BJT input stage
In the BJT input stage, the base current is required in the input transistors which allows the collector current to flow. The range of this current could be from a few nanoamperes to some microamperes. BJT input stage amplifier needs to be driven by sources having relatively low output impedances.
CMOS input stage
In the CMOS input stage, there is no input bias current because the transistors have an oxide barrier at the gate which is like an open circuit. These amplifiers are used to read out voltages where the source’s output impedances are relatively high like sensors.
Input offset current
Input offset current, also known as input bias current imbalance, is a parameter associated with operational amplifiers (op-amps) and is closely related to input bias current. Input offset current represents the difference in the bias currents between the two input terminals (inverting and non-inverting) of an op-amp.
Input offset current can manifest itself as offset voltage if the effective resistance at its inverting and non-inverting pins are not matched.
Slew rate
Slew rate is defined as the maximum rate of change of the output voltage concerning time. It is expressed in volts per microsecond (V/µs). The large signal step response is an indication of the amplifier’s slew rate.
An opamp has an internal transconductance stage (gm) which takes the input differential voltage and converts it to an output current, Iout. Iout flows into the Miller compensation capacitor (Cc). During slew, the feedback loop breaks because the amplifier is not fast enough to respond. This means, that for the duration of the slew, vip – vim is large and constant. We know that Iout is gm(vip-vim). If Iout is a constant, then the voltage across Cc will rise linearly with time. This is called slew.
Common mode rejection ratio
CMRR is a related parameter that specifies how well the op-amp rejects common-mode signals. It is expressed in decibels (dB) and represents the ratio of the differential gain (the gain for signals applied between the inputs differentially) to the common-mode gain (the gain for signals applied to both inputs simultaneously). A high CMRR indicates good common-mode rejection.
Common mode input range
This specification defines the minimum and maximum voltages that can be applied to both the inverting and non-inverting inputs simultaneously while ensuring that the op-amp operates within its specified common mode rejection ratio.
Output voltage headroom
The output voltage headroom of an operational amplifier (op-amp) refers to the maximum voltage difference between the op-amp’s output voltage and the voltage levels it can generate without distortion. The maximum voltage an opamp can theoretically reach is the supply voltage. However, in real opamps, depending on the architecture of the output stage of the opamp, there is a difference between the theoretical maximum (supply voltage) and real output saturation level. As soon as the output is near saturation, distortion increases a lot.
Power supply rejection ratio
The power supply rejection ratio (PSRR) measures the ability of an op-amp to reject variations in its power supply voltage, meaning it quantifies how well the op-amp can maintain a stable output when the power supply voltage changes.
The PSRR is measured as a ratio of the change in the input offset voltage (Vos) or output voltage (Vout) to the change in the power supply voltage (Vcc/Vdd or Vee/Vss), usually in decibels (dB).
$$PSRR+=20log_{10}\left(\Delta{}V_{out}/\Delta{}V_{cc}\right)$$
$$PSRR-=20log_{10}\left(\Delta{}V_{out}/\Delta{}V_{ee}\right)$$
Noise or Input referred noise
The noise generated within the op-amp can be referred back to the input due to the amplifier’s gain. This is commonly expressed as input-referred noise voltage or current, which allows engineers to assess the impact of op-amp noise on the input signal.
- Thermal Noise (Johnson-Nyquist Noise): This type of noise arises due to the random thermal motion of electrons within the resistors and semiconductor components of the op-amp. It is directly related to temperature and resistance and is characterized by a continuous spectrum of frequencies. Thermal noise is also known as white noise because it has a constant power density across the frequency spectrum.
- Shot Noise: Shot noise occurs when current flows throughout a PN junction. It arises because current consists of a vast number of discrete charges, and is not totally continuous. The continuous flow of these discrete pulses gives rise to almost white noise. There is a cut-off frequency which is due to the time it takes for the electron/holes to travel through the PN junction. Shot noise density is uniform across frequencies.
- Flicker Noise (1/f Noise): Flicker noise, also known as 1/f noise, is characterized by a spectrum where noise power increases as the frequency decreases. It is caused by dangling bonds near 2 dissimilar material junctions or semiconductor surfaces. Flicker noise can be particularly problematic in low-frequency precision applications.
- Burst Noise (Popcorn Noise): Burst noise consists of intermittent, sudden variations in voltage or current. It can be caused by defects in the semiconductor material and is more prominent at high frequencies. It appears as bursts of noise with random durations and amplitudes.
Distortion / THD
Distortion refers to the deviation of the output signal from the input signal. Op-amp distortion can affect the accuracy and fidelity of amplified signals in electronic circuits.
Distortion can be categorized into the following:
- Harmonic Distortion: Harmonic distortion is one of the most common types of distortion in op-amps. It occurs when the input is an ideal sinusoid and the op-amp introduces additional harmonics or frequency components at the output.
- Intermodulation Distortion (IMD): Intermodulation distortion occurs when two or more different frequency signals are present at the input of the op-amp. The op-amp can mix or modulate these signals, creating sum and difference frequency components that were not in the original signal. IMD can lead to the generation of unwanted spurious frequencies near the input signal frequency which are hard to filter out.
- Slew Rate Limiting: When an op-amp’s input signal changes too rapidly, and the slew rate of the op-amp is insufficient to follow the input. If a high-frequency sinusoid is given at the input, the output waveform becomes triangular instead of sinusoid.
- Cross-Over Distortion: Cross-over distortion is specific to push-pull or complementary output stages of op-amps, such as those found in audio amplifiers. It occurs at the transition point when switching between the two output devices (e.g., transistors), leading to distortion near the zero-crossing point of the input signal.