Operational Amplifier: Characteristics and Compensation
Operational Amplifier Characteristics and Compensation
Feedback
Feedback reduces entry, thus affecting the output signal.
Advantages of Feedback:
- Front-end to stabilize the gain components variations in LS or LS-polarization.
- Improved sources of LS of the resistances of entry characteristics and increased output bandwidth.
- Wider bandwidth.
- Reduced flow series of noise to the output signal.
Disadvantages of Feedback:
- The gain with feedback will be less than the gain without feedback.
Compensation Methods
Compensation consists of introducing RC below or varying the amplifier so that the characteristics of the module and phase of loop gain are adjusted.
Dominant Pole
(R = 1/2?RC series C) Introduction of 1 pole at a frequency that is less than the pole. The poles open up the 0dB (fp1 and f1). The other 2 (f2 and f3) decrease DCAD, remaining below 0dB.
Cross Over
(R1 || R2, C series R2: for = 1/2πR1C; fp = 1/2π(R1 || R2) C) Introduction of 1 pole and 1 zero, the frequency of the zero being less than the pole. The zero is set to equal f2 and fp is set above 0dB for stability.
Pole-Zero
(R1 series R2 series C; for = 1/2πR2C; fp = 1/2π (R1 + R2) C) Introduction of 1 pole and 1 zero, the frequency of the zero now being higher than the pole. The zero is set to equal f1 => f2 and fp only do dB above.
Direct Coupling
Direct coupling joins levels, which is normally used to enter complementary voltage sources. It has good low-frequency response and is used in integrated circuits.
Capacitive Coupling
Capacitive coupling means the continuous component of one amplifier does not pass to another. The impedance of the capacitor is necessary, acting as a short within the frequency range of operation.
Stability Criteria
For a system to be stable, the gain module must be less than unity when the phase is 180º / below / <1 to -180º
Gain Margin (GM)
The gain margin is the module of loop gain at which the frequency phase is -180º. If the gain margin is below 0dB, the system will be stable. It indicates how much the gain can be increased while the loop system remains stable (as long as it stays below 0dB). If it is above 0dB, the system will be potentially unstable.
Phase Margin (PM)
The phase margin is the difference between -180º and the frequency phase at which the module of loop gain is 0 dB.
Typical Values:
- GM: 10dB
- PM: 50º
Virtual Short Circuit
If the output voltage is finite, and the gain is very high, the voltage difference between the entry terminals must be very small (0). Therefore, there is a virtual short circuit of effect between the input terminals. Vo = A (V2-V1) => V2-V1 = V / A = 0 => V2 = V1
Virtual Ground
Setting V2 = 0, there is a virtual ground at the 1st input terminal. V2 = 0, V2 = V1, V1 = 0.
Common-Mode Rejection Ratio (CMRR)
CMRR = |Ad| / |Ac| serves to measure the degree of deviation from ideal behavior.
Ideal Op-Amp Characteristics
Large Ad, small Ac, large bandwidth, small input current, small input offset voltage, large input impedance, small output impedance. In Darlington configuration, there is high input impedance, and small power supply current.
Op-Amp Specifications
- Power supply: symmetrical (+-5, +-15).
- Maximum differential and common-mode input voltage. Example: 30V.
- Dissipation: maximum power dissipation.
- Operating ambient temperature for safety.
Slew Rate
Slew rate is the maximum rate of change of the output voltage with respect to time (V/µs). Output signals are always extensive. To measure it, apply a large amplitude rectangular signal at the input of a voltage follower. SR = min (V/T1, V/T2).
Bandwidth
Full power bandwidth or maximum frequency for a sinusoidal signal of maximum amplitude at the output. Distortion begins to show due to slew rate. The signal from fmax shows distortion at the output due to slew rate.