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Analog Techniques
Published in Gillian M. Davis, Noise Reduction in Speech Applications, 2018
A technique that can be used to give precise control of the output dc conditions is to incorporate an analog servo amplifier in the feedback path of a negative feedback control loop. The transresistance amplifier shown in Figure 2.2(b) includes a noninverting servo amplifier. A servo amplifier is normally a single linear integrator (i.e., gain inversely proportional to frequency, −90° phase shift) with a suitably large time constant so as only to influence the closed-loop gain at low frequency. At signal frequencies approaching dc, the servo amplifier should have an extremely high gain (typically 105 for an operational amplifier) that then forces the closed-loop gain of the transresistance amplifier to become virtually zero at dc. If the servo amplifier dc gain is assumed to be infinite, then the output voltage of the transresistance amplifier is controlled by negative feedback so as to have an average value (i.e., the quiescent dc value) that is equal to the dc input offset voltage of the servo amplifier. Normally with operational amplifiers such as a BI-FET,* the input bias current is negligible and the input offset voltage is typically under 1 mV. A servo loop can accurately maintain the average output voltage close to zero enabling dc coupling to be used at the output of the transresistance amplifier, where the closed-loop transfer function now includes a first-order high-pass filter response. The servo amplifier can achieve a low value of cut-off frequency without recourse to high-value capacitors, as the input resistors in the servo amplifier that are instrumental in determining the integrator time constant can be large.
Application of Operational Amplifiers
Published in Bogdan M. Wilamowski, J. David Irwin, Fundamentals of Industrial Electronics, 2018
Carlotta A. Berry, Deborah J. Walter
Unlike an ideal op amp, when the input voltage to the practical op amp is zero, the output is not zero. The input offset voltage is the differential input voltage that is needed to make the output zero when the input is zero. A typical value for the input offset voltage is 2 mV. The output when the input is zero is the output dc offset voltage. The way to compensate for this value in a realistic design is to add a small voltage source at the inverting or non-inverting amplifier of the opposite magnitude and polarity. It may be necessary to use a potentiometer with the op amp input set to zero to find the exact value to cancel out the input offset voltage.
Differential and Operational Amplifiers
Published in Nassir H. Sabah, Electronics, 2017
As in the case of the BJT, an input offset voltage Vio arises from a mismatch between the two halves of the circuit. However, Vio is considerably larger for the MOSFET case, compared to its BJT counterpart, in the ratio of Vov/2 to VT. It can be readily shown that: If the two RD resistors differ by ΔRD, the input offset voltage Vio is (Problem P10.1.21):
Voltage-controlled oscillators using CFOAs with correction terminal Z
Published in International Journal of Electronics, 2023
According to the operational principle, the CCII+ can be considered as equivalent to a BJT or MOSFET (Sedra et al., 1990), the difference between the two elements being that, at a voltage Vyx >0, the amplifier can source current iz, while for Vyx <0 – can sink a iz. At this case, depending on the value of the current ix, the value and direction of the iz is also obtained, i.e. in the circuit of the CCII+, conversion of the current into the current is obtained by using a current-controlled current source (CCCS). Unlike the positive current conveyor at the nMOSFET, can only be sunk and that at the threshold voltage VGS > VTh. An advantage of the CCII+s is that in the bias point the voltage between y and x is approximately equal to zero. This value is determined by the input offset voltage and is usually on the order of a few millivolts. To illustrate the above description, the block diagram of a CCII+ and its corresponding nMOSFET are shown in Figure 1.
A low-power, low-offset, and power-scalable comparator suitable for low-frequency applications
Published in International Journal of Electronics, 2023
Riyanka Banerjee, M. Santosh, Jai Gopal Pandey
The conventional power-efficient latched comparator does not have a static current throughout the operation. The main drawbacks are the limited input range and kickback noise, as mentioned in Miyahara and Matsuzawa (2009), Schinkel et al. (2007), Lan et al. (2011). An inverter-based comparator is reported where the input range is rail to rail and operates at moderate speed Kumamoto et al. (1986), Lee et al. (2004). The reported comparator consists of a sampling stage and an amplifier stage followed by digital drivers. The amplifier stage consists of cascaded inverters. The mismatch between the switching threshold voltage of the cascaded inverters contributes to the input offset voltage of the comparator. The input offset is controlled by inserting capacitors and switches into each stage, which requires additional control signals.
A full input range, 1–1.8 V voltage supply scalable analog voltage comparator in 180nm CMOS
Published in International Journal of Electronics, 2021
Ashima Gupta, Anil Singh, Alpana Agarwal
The input offset voltage is the important design metrics in the comparator design. In the proposed comparator, the foremost cause of the offset voltage is the difference in rising (falling) time delays between td_r1 and td_r2 (tf_r1 and tf_r2) of digital buffers in the upper and lower arm where td_r and tf_r represent the rising time delay and falling time delay, respectively. The variation in rising and falling delay leads to the offset voltage, where is the current flowing through the load and . Also, the mismatch in the switching threshold voltages of inverters INV1 and INV3 at the input side causes the offset voltage, i.e.. Also, MOSFET-based resistance, which has a random variation, is given by