An Adaptive Biasing Technique to Convert a Pseudo-Class AB Amplifier to Class AB

An essential modification to a conventional four-stage pseudo-class AB amplifier results in a true class AB amplifier. As opposed to the conventional pseudoclass AB scheme, which uses a current mirror in the third stage, our work uses an adaptive biasing circuit in the third stage in order to drastically reduce quiescent current. The pseudo-class AB and new class AB amplifiers are designed in a 0.5μm process with power supplies of ±1.5V for a phase margin of 67 o while driving a load of 32Ω || 500pF. At a maximum negative load current of −12.5mA, simulations show that both amplifiers consume 43.9 μA of quiescent current. However, at a maximum positive load current of +12.5mA, the pseudo-class AB consumes 1.22mA of quiescent current, while the new class AB amplifier consumes only 140μA, a reduction in quiescent current by more than a factor of eight at maximum positive load. Indexing terms: Analogue circuits, Adaptive biasing, multistage amplifiers, class AB amplifier, pseudo-class AB amplifier, pole tracking. As battery-powered circuits become ubiquitous, the demand for highly current-efficient amplifiers and drivers is increasing in order to reduce the size and/or Introduction: increase the lifetime of the battery [1]. However, as the dimensions of transistors become smaller, the inherent gain of each transistor stage is reducing. In order to achieve high gain, multistage amplifiers are required. High current efficiency is generally achieved by employing a class AB push-pull output stages. As a result, multistage class AB amplifiers are among of the most widely used circuits in battery-powered applications. A low-voltage [2] four-stage pseudo-class AB four-stage amplifier has been widely reported in the literature [3]–[5]. Its schematic is given in Fig. 1(a). This pseudo-class AB amplifier requires a large amount of bias current whenever the output load current is large and positive due to the current mirror formed by transistors M13–M15 between the third and forth stages. On the other hand, when driving a large and negative output load current, the bias current remains low. The designation pseudo-class AB results from the fact that the maximum positive output current is proportional to the bias current of the third stage. To overcome the problem of a large bias current in the third stage, we are proposing an adaptive biasing circuit in the third stage, as shown in Fig. 1(b). This essential change converts the output stage to true class AB, such that the maximum output current is much larger than the bias current [6]. This adaptive biasing circuit also known as pole-tracking, finds use in low dropout voltage regulator applications [7]–[9]. . Improved Class AB Topology: Referring to Fig. 1(b), the input stage is implemented using a folded-cascode amplifier with transistors M1–M9. Transistors M10–M12 realize a common-source gain stage. The third stage M13–M14 also realizes a common-source gain stage with the proposed adaptive load. The fourth stage is the push-pull output stage, formed by M15–M16. The third stage bias current adapts so as to boost the output load current when a large positive input signal is applied, yielding a maximum positive load current much larger than the quiescent current. The adaptive load of stage 3 consists of a diode-connected transistor M13 and resistors Rad1 and Rad2. At very low output currents, transistor M13 is in cutoff and the stage 3 load is simply Rad2. The presence of parallel resistor Rad2 helps move the non-dominant pole at the gate of M15 to higher frequencies and improves the overall phase margin of the amplifier [8]. At higher output currents, M13 goes into saturation and the third stage load resistance becomes (1/gm13+Rad1) || Rad2. The presence of Rad1 in the source of M13 forms an inverseWildar current mirror between M13 and M15 [10]. Depending on the choice of resistor values, the output resistance of third stage of the proposed class AB amplifier can be much higher than that of the conventional pseudo-class AB amplifier, which is 1/gm13. As a result, the overall gain of the proposed class AB amplifier can be proportionately higher, as well. Simulation Results: The circuits in Fig. 1 were simulated in 0.5μm 2P3M CMOS ON Semi process with HSPICE using device dimensions given in Table I. The push-pull output stage has dimensions that are 6x to 12x than that of the first three stages, so as to be able to sink and source large load currents. Compensation networks were designed for each amplifier such that a phase margin of 67 o was achieved in both cases. The true class-AB amplifier required an additional Miller capacitor with nulling resistor (Rm1 and Cm1 in Fig. 1(b)) so as to achieve the desired phase margin. The amplifiers were configured as inverting amplifiers with a gain of negative four, driving a load of 32Ω || 500pF. Fig. 2 shows transient simulation results of both the pseudo-class AB and class AB amplifiers where the input is a square wave of ±100mV with rise and fall times of 10ns. At a maximum negative load current, ILMAX−, of −12.5mA, both amplifiers consume 44μA of quiescent current. However, at a maximum positive load current, ILMAX+, of +12.5mA, the pseudo-class AB consumes 1.22mA of bias current. On the other hand, the true class AB amplifier consumes only 140μA for the same output load current. A summary of bias currents at different load currents is summarized in Table II. The current boosting factor (CBF), defined as ILMAX / IQ in [1], is 89 for the proposed amplifier, compared to a value of 10.2 for the conventional pseudo-class AB amplifier, for a positive load current of 12.5mA. A performance comparison of the two amplifiers is given in Table III. The major difference between the two amplifiers is the gain, which improves from 53.7 dB for the pseudo-class AB amplifier to 71.9 dB for the class AB amplifier. As described earlier, this increase is due to the higher load resistance of stage 3. Conclusions: A simple but effective scheme for converting a pseudo-class AB amplifier to a true class AB amplifier is introduced. The adaptive biasing circuit results in a lowvoltage, low-bias current amplifier. Simulation results illustrate the improved operation of the proposed class AB amplifier.