Class AB power amplifier driver circuit research and design

1 AB class amplifier drive circuit design goals

In practical circuit, often require the final stage amplifier (ie output level) a certain power output to drive the load. Able to provide sufficient signal power to the load the amplifier circuit as power amplifier circuit, referred to as amplifier. There are 4 types of classical PA: A class, AB class, B class and C class, their main difference is that the bias is different. 4 class classic amplifier ideal maximum efficiency of the theoretical value and function of conduction angle as shown in Figure 1.

Class AB power amplifier driver circuit research and design

A class of power amplifier linearity, power transmission ability, the maximum efficiency of 50%, conduction angle of 360 °; B power amplifier transistor by reducing the cycle time to increase efficiency of work (preferably up to 78.5%), maintaining the possibility to achieve linear modulation, duty cycle for the half-cycle; C class power amplifier provides a nearly 100% efficiency, but at the same normalized power transmission capacity and power gain are zero, the line Sexual Difference; AB class amplifier efficiency and linearity in the A class and B class amp, its greatest feature is the conduction angle in the range 180 ° ~ 360 °, the corresponding design goal is to realize he was in a cycle 50% and 100% within a certain period of time between the conduction of the work, the single-MOS pipe, it means that he's drain current is passed through more than half a period of time.

2 amplifier drive circuit design and simulation of specific

2.1 The mirror current bias mode

In dual power supply of the differential amplifier, the two static working point current directly from the constant current source circuit. On the constant current source bias circuit requirements, in addition to providing a stable quiescent operating point current, it should also have high output AC resistance. Mirror constant current circuit is the most widely used of a high stability constant current source circuit, he is particularly suitable for use in integrated circuits. Figure 2 is to be achieved by mirroring the bias current of the drive circuit diagram.

This circuit is from 2 to strictly match the personality of NMOS tube and a resistor, an inductor component, IM1 and IM2 were two NMOS control circuit drain current of M1 and M2. M1 and M2 control the substrate and the source of a short tube connected, there is no body effect. Since the two NMOS control exactly the same width to length ratio, therefore, AB class power amplifier driver circuit research and design changes in VDD or R, IM1 and IM2 corresponding also changed. Given IM1 IM2 like a mirror, it will be such a constant current source circuit called mirror constant current source circuit. Figure in front of C and L with the partial pressure of the role of bias discussed in the same way.

When the two are completely symmetrical, the temperature will not lead to changes in IM1 and IM2, the mirror current source circuit is a heat stable bias circuit. This method also eliminates the bias voltage with a fixed gate bias related to the thermal drift.

The class AB amplifier, for a given VDD is 3 V, Vin for the DC bias 2 V, the amplitude of 1 V, frequency of 1 GHz sine wave, R selected for the 800 Ω, C is 0.5 pF, L to 0.065 nH, M1 and M2 are wide and 0.6μm, length 0.18 μm for NMOS. From Figure 3, the drain current of transistor M2 HSpice simulation waveform can be seen Vg ≥ 0.297 V the length of time for the 0.69 ns, greater than 0.5 ns of the half cycle time, thereby achieving the AB class amplifier drive circuit requirements work longer than half a cycle.

Class AB power amplifier driver circuit research and design

2.2 divider bias method

Partial pressure of type bias circuit, the name suggests is through the resistor divider to be achieved given the bias voltage circuit shown in Figure 4.

Class AB power amplifier driver circuit research and design

Circuit for the DC blocking capacitors C, isolated from the input signal DC component caused by a variety of reasons, to ensure the circuit characteristics of DC component is not affected by the accident. The power supply circuit generally through choke L to the MOS drain tube feed, the purpose is to reduce the unnecessary DC power circuit loss and improve power amplifier efficiency in low voltage power output greater . Thus, the circuit will choke inductor L connected to the drain power supply and between the M1, will be sent to DC power MOS tube drain.

Circuit for the source resistor R3, and its value is small, so consumed in his above DC power consumption is very small, to minimize the DC circuit unnecessary power loss and improve power amplifier efficiency. R1, R2 are called the bias resistors and resistor under bias, their role is to carry out partial pressure of VDD, the MOS gate control gate to generate a static voltage Vg, the value is:
Partial pressure of type bias circuit not only effective in stabilizing the quiescent point, but also for different transistors, because of parameter inconsistency arising from the static operating point changes. Also has an automatic regulation.

The class AB amplifier, for a given VDD is 3 V; Vin for the DC bias 2 V, the amplitude of 1 V, frequency of 1 GHz sine wave, selected R1 is 2 kΩ, R2 is 1 kΩ, R3 is 10 Ω, C for the 5 pF, L to 0.065 nH, M1 for the width of 0.6 μm, 0.18 μm length of the NMOS, NMOS from the given parameters can be calculated Vth0 about 0.297 V. Set the resistance so Vg 1 V or so up and down swing around, swing is 1 V, transistor M1 can make working time more than half of the cycle is less than the entire cycle, from Figure 5 HSpice transistor M1 drain current waveform simulation also can see that the weekly working time of 0.79 ns, more than half of cycle, to achieve a class AB amplifier drive requirements.

Class AB power amplifier driver circuit research and design

2.3 gate diode bias means

Shown in Figure 6, this circuit is a partial pressure of the special structure, through the resistor, inductor, diode on the VDD to partial pressure, the M1 is the gate to generate the static bias voltage Vg was higher than during the week 0.297 V part of the increase, so that you can achieve class AB amplifier bias-driven requirements.

Class AB power amplifier driver circuit research and design

Given VDD is 3 V, Vin for the DC bias 2 V, the amplitude of 1 V, frequency of 1 GHz sine wave, selected C to 15 pF, R is 10 Ω, L1 is 0.065 nH, L2 is 2 nH, M1 is width 0.6μm, length 0.18 μm for NMOS. HSpice simulation shows, the work within 1 ns time 0.75 ns, to achieve a class AB power amplifier driver circuit design goals, hours of work per week, more than half of cycle.

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