20th Iranian Conference on Electrical Engineering, (ICEE2012), May 15-17,2012, Tehran, Iran

Class AB Amplifier with Simultaneous Amplitude and Output Current Tracking

Soroosh Rasty Boroojeni, Mojtaba Sadrpour, Ehsan Adib, Member IEEE sorooshrasty@gmail.com, seyedsadrpour@gmail.com, e.adib@cc.iut.ac.ir

Department of Electrical and Computer Engineering, Isfahan University of Technology

Abstract: Tn this paper a new technique is introduced for

simultaneous amplitude and output current tracking of class

AB amplifier. In the proposed circuit, the output voltage is

provided by a class AB amplifier benefiting from the linear

characteristic of this amplifier. However, the load current is

provided by a class D amplifier in order to increase the

efficiency. Furthermore, the class D amplifier provides the

supply voltage of class AB amplifier with the capability of

amplitude tracking. The proposed technique is simulated

using PSpice software. Simulation results show efficiency

improvement of at least 6% in comparison to current

tracking technique. Also, experimental results are presented

to justify the validity of theoretical analysis and simulation

results.

1- INTRODliCTION

Class AB has the advantages of linearity and wide band operation. Also, the phase delay between input and output signal is zero in this amplifier. However, the efficiency of this amplifier depends on the amplitude of the output signal and decreases at low output voltage swings. Class AB amplifiers are usually implemented using BJT transistors. For high power applications, class AB amplifier is not proper choice due to high power losses of the power transistor. Class D amplifier which is usually implemented using MOSFET transistors operates based on switching the power transistor between triode region and off region. Therefore, the power losses of the power transistor reduce to much extent. In class D amplifier the output signal is obtained from filtering the high frequency harmonics of the PWM

(0)

Linear 20 � ......... RFI'A �

(e)

signal. Therefore, class D amplifier suffers from nonlinearity due to switching ripple at the output voltage and also, high THD due to the required dead time for driving the switches. Furthermore, the output voltage lags the input voltage due to filter frequency response. Several methods are provided to improve class AB efficiency using class D power amplifier in the literatures [1-11]. For radio frequency applications, signal envelope is tracked using class D amplifier [3]. The schematic of this method is shown in Fig. 1. This method, guarantees that the efficiency of class AB amplifier remains maximum which is approximately equal to 60% at any output voltage swing.

Another method which is introduced in the literatures is based on output current tracking [1,2]. In this method, the load current is provided using class D power amplifier while the output voltage is forced using class AB amplifier. The schematic of this method is shown in Fig. 2. Tn this method, Rsense, senses the output current of class AB amplifier and controls inductor L current using a class D (buck converter) amplifier in such a way to reduce the Rsense current to zero. This method is adopted for audio amplifier applications in [1]. In this method, due to presence of Rsense which is in series with the output voltage of class AB and since this resistor experiences pulsating current, the load voltage experiences a very small switching ripple.

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(b)

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ma .. � imutn efficiency region

(d)

Variable "QItll,gC

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Fig . 1- envelope Tracking method for class AB amplifier[3].

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Envelope signal

'V

Fig. 2- Current tracking for improving class AB amplifier [1].

Tn this paper, class D or buck converter is used to provide the load current. Tn order to prevent from switching noise which presents across the load due to Rsense, the place of this resistor is changed. Furthermore, the mentioned buck converter is also used to provide a voltage similar to signal amplitude which is used as the power supply for class AB amplifier. In this way, the transistors applied in the class AB amplifier operate under low voltage and low current condition. These results in higher efficiency and much lower power rating of class AB transistors and at the same time a remarkable reduction in switching frequency of class D stage can be achieved which simplifies the practical implementation. Simulation results using PSpice software and also

+Vcc RIVE CIRCTJ T

-=-+Vee .l

-=-0

experimental results are presented which justifies the effectiveness of the proposed technique.

U- PROPOSED OUTPUT CURRENT TRACKING

TECHNIQUE

The schematic of the proposed load current tracking technique is shown in Fig. 3. In this circuit, the Rsense resistor is placed in the collector of class AB transistors. Thus the load voltage is actually the output voltage of class AB amplifier and thus the output voltage does not contain any switching harmonic and benefits from linear advantages of class AB amplifier.

M1

D biais L1

D bias

-Vee

.l -Vee -=-0

D1 R Load

M3

RIVE CIRCTJ T

-=-0

-:::=- + ouP-----------�

COMPARATOR Fig. 3- Proposed current tracking technique for class AB.

359

D2

M4

-=-0

When the output voltage and output current of class AB is positive, Ml and Dl are used to control the L current. In this condition, once Rsense voltage reaches a threshold, M J is turned on to increase inductor L current. Inductor L current is:

V -V 1 = CC In I1t + 1 L L

LO (1)

This results in the reduction of Q 1 current. Once the current of Rsense decreases below the threshold, MJ is turned off and D I conducts L current. In this condition inductor L current is:

1 -�n A L = --D.t + ILl (2) L

In this mode Mz is always on. Also, when the output voltage is negative, M3 and Dz are used to control the L current. For proper operation of the circuit, rate of inductor L current increment at switch tum on time should be higher than rate of output current increment at any condition. Thus

�:c: Am -- >--2L - R load

(3)

Where A is the maximum amplitude of load voltage and w is the maximum angular frequency of load voltage. Inductor L can be chosen according to the above equation. Also, over design is required in selection of L to guarantee the proper operation of the circuit. The circuit applied for sensing Rsense voltage and driving M 1 gate is shown in Fig. 4. In the proposed current sensing method, since the voltage ripple across Rsense is not appeared in the output, the bang-bang control loop that controles inductor L current can be selected wider. Therefore, the switching frequency of MJ decreases to much extent and the efficiency of the circuit increases. The main problem of this method is at low output voltage swings. In this condition, the load current is low and therefore, inductor L switching ripple is comparable to amplitude of load current. Thus, due to inductor L ripple, the power losses of class AB amplifier are relatively high.

• Vlhreshol

R Sense M1

""0

Fig. 4. Gate drive circuit.

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Once the switching frequency is not much higher than output frequency or in other words the inductor L is relatively large, Inductor L current can not immediately track the output current at zero crossing condition which forces high peak power to class AB transistors. In this condition, in order to improve class AB efficiency, its operating voltage should be reduced. Therefore, the amplitude of signal can be tracked and this voltage can be applied as the supply voltage of class AB amplifier. Also, this technique is effective to further reduce the total losses of the amplifier at high output voltage swings.

TII- SIMlJLTANEOlJS OlJTPlJT ClJRRENT AND

AMPLITlJDE TRACKING

The proposed circuit for simultaneous tracking of output voltage and load current is shown in Fig. 5. In this circuit when the output voltage is positive, MJ and DJ are used to control inductor LJ current just like the current tracking method discussed in the previous section. Furthermore, D3, MJ, D4, Lz and CJ create a buck converter which is preferred to operate in Discontinues Conduction Mode (DCM). The voltage across C1 which is the output voltage of buck converter, supplies the class AB power transistor. The voltage across CI is proportional to the converter duty cycle. According to volt-second balance of inductor LJ, the converter duty cycle should increase as the output voltage increases.

D= !i V:c

(4)

Where D is the operating duty cycle of buck converter, Vee is the supply voltage and Vo is the instantaneous output voltage. Thus, the supply voltage of class AB amplifier increases proportional to signal amplitude and therefore, the voltage across the class AB power transistor decreases which will further increase the amplifier efficiency. DCM operating mode is selected for the buck converter to have fast voltage response in order to track the amplitude of output voltage properly. In this circuit, when the output voltage is negative, according to the value of inductor LJ current, the NPN or PNP power transistor of class AB amplifier may conduct. In this condition if the NPN transistor conducts, since MI is off, C1 voltage can become negative. The small current source II which can be easily implemented using a low power transistor, prevents CI voltage to become negative when the output voltage is negative. Negative voltage across CJ will be problematic once the output voltage should become positive. In other words, CJ voltage should be positive at the beginning of the next positive output voltage cycle for proper operation of the circuit.

L2 . ,

D4 _ C1

+Vee • I

+Vee

D biais1iL

D bias2iL

-v�

-v�

D3

R Load

+Vee -Vee = +Vee

M1

M3

COMPARATOR

COMPARATOR

-Vee - 0

D2

M4

Fig. 5. Proposed output current and amplitude tracking technique.

IV- SIMULATION RESULTS

The proposed voltage and output current tracking technique is designed for a 12.5W class AB power amplifier for the application of audio amplifier. The value of Ll, L2 and Cl are 75uR, lOuR and 820nF respectively. The simulation results are shown in Fig. 6 - 9. In figure 6 and 8, the load voltage, capacitor C1 voltage and capacitor C2 voltage are presented. The output frequencies in these figures are 4kHz. It can be observed that C I and C2 voltages are tracking the output voltage which results in reduction of voltage across power transistors of class AB which causes to improve peak power loss of BJTs by more than 50% over previous method.

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,/ /"

/ V "� /

Tn figure 7 and 9, inductor L current and output current are shown and in figure 10 the efficiency of proposed technique is shown for different output voltages and is compared with the current tracking technique. According to simulation results, efficiency of proposed technique is at least 6% higher than current tracking technique. At low output voltage swings, the proposed technique results in approximately 20% efficiency improvement in comparison to current tracking technique at the same switching frequency. This remarkable improvement of efficiency at low power outputs is mainly due to reduction of BJT power losses which become dominant at current tracking technique for low voltage outputs and results in efficiency degradation.

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� ,

''--..........-.. V'/ C..----� ------- ./

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Fig6 . . Capacitor Cl voltage (top waveform), output voltage (middle waveform) and capacitor C2 voltage (bottom waveform).

For 4kHz input. (Vertical scale 5V/div, Time scale 20us/div)

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Fig. 7. Inductor Ll current and output current. For 4kHz input. (Vertical scale lA/div, Time scale 20us/div)

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� --""'---� --

.-/ "", /" , /

'- A .,

" / � '.../ \. � � 1------,././�

�

,t' VV'" w}\l'/V' V

Fig8. Capacitor Cl voltage (top waveform), output voltage (middle waveform) and capacitor C2 voltage (bottom waveform)

for output voltage amplitude of 3v and frequency of 4kHz. (Vertical scale 2V/div, Time scale 20us/div)

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�' '� / � I rf

� � \fi� V

'" V%i\ VW/IN �// "'------. �

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Fig. 9. Inductor L I current and output current for output voltage amplitude of 3v and frequency of 4kHz. (Vertical scale O.4A/div, Time scale

20us/div)

09

0.8

is 0.7 -• . " � 0.6-

05 -

04 0.2 0.4 0.6 0.8

Signal amplitude normalized to Vee

Fig. 10. Efficiency of proposed teclmique (dashed line) versus current tracking technique (continuous line).

V. EXPERIMENTAL RESULTS

A prototype of the proposed amplitude and output current tracking technique is implemented. The schematic of the implemented circuit is shown in Fig. 11. The implemented circuit is for positive output half cycles. The value of inductors L 1, L2 ,R load and

362

capacitor C in the implemented circuit are 75uH, lOuH,lOohm and 470nF respectively. The experimental results are shown in Figures 12, 13 and 14 respectively which justifies the validity of theoretical analysis and simulation results.

VI. CONCLlISIONS

Tn this paper a new technique is introduced to simultaneously track the output current and amplitude of class AB amplifier. The class AB provides the output voltage. However, the output current is provided by class D amplifier. Furthermore, class D amplifier provides a voltage similar to the output voltage which is used as class AB supply to further improve the efficiency. The proposed technique increases the efficiency especially when the output amplitude is low. The simulation results show at least

6% efficiency improvement with respect to current tracking technique.

D3 +Vcc <:::>-_-KJ----1r---....,...L1 fYL2yy ...... --IKI----,

D2

..b e �o

L 1

R Load

�o Fig. II. Schematic of implemented circuit.

II Trig'd M ..

+Vcc

M5

�o

Fig. 12. Supply voltage of class B amplifier (top waveform) and

output voltage (bottom waveform) for output amplitude of 5.5V and output frequency of 4kHz.

Fig. 13. Supply voltage of class B amplifier (top waveform) and

output voltage (bottom waveform) for output amplitude of 2V and output frequency of 4kHz.

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