Low voltage push-pull amplifier
This invention relates to amplifiers, and particularly push-pull amplifiers arranged to operate in Class B at low supply voltages. 
     With the desire in the electronics industry for the miniaturisation of components and devices, and the public desire for miniature, but efficient communications devices, an amplifier capable of driving loads such as loudspeakers at low voltages is clearly of interest. We have devised such amplifiers, which are suitable for use with supply voltages of 1 to 2 voltages (typically available from electronic watch batteries) and which use a minimal number of components. The circuit is designed for Class B push-pull operation and therefore is especially suited for use in the output amplifier stage of a miniature radio receiver, or low power hearing aids. 
     According to the invention we provide a complementary push-pull amplifier comprising: 
     a series connection between the power supply rails of first, second and third resistors, the first resistor being connected to the second resistor through the emitter and collector of a first transistor, the second resistor being connected to the third resistor through the emitter and collector of a second transistor, the bases of the first and second transistors being connected together to receive an input signal for amplification; 
     a pair of complementary transistors connected in series between the power supply rails with the respective bases receiving signals from opposite ends of the second resistor; and 
     means for providing an amplified output from the junction of said complementary transistors. 
    
    
     Preferred features of the invention will now be described with reference to the accompanying drawings, in which: 
     FIG. 1 is a circuit diagram of a preferred amplifier according to the invention; 
     FIG. 2 is a circuit diagram showing a preferred configuration for the amplifier of FIG. 1; and 
     FIGS. 3 and 4 are similar to FIGS. 1 and 2, but show an integrated circuit implementation of the invention. 
    
    
     Referring to FIG. 1, first, second and third resistors R 1 , R 2 , and R 3  are connected in series between the power supply rails 2 and 4 and interconnected by complementary transistors T 1  and T 2 . A complementary pair of output transistors T 3  and T 4  have their bases connected to opposite ends of R 2 . The bases of T 1  and T 2  are connected together and receive the signal from an input transistor T 5 . The collector of T 5  is biased through resistors R 4  and R 5 , whereas the input to the amplifier, at 6, is through resistor R 6  to the base of T 5 . A feedback from the junction of T 3  and T 4  is provided through resistor R 7  to the base of T 5 . The amplified output for the circuit is also taken from the junction of T 3  and T 4 , through capacitor C and loudspeaker L. 
     The circuit is operated with a supply voltage of about 1 to 2 volts and in the Class B mode with silicon bipolar transistors. It has a stable output bias at low voltage. The output quiescent current is adjustable, if required, by trimming the value of R 2 . Preferably R 1  and R 3  have substantially the same value and R 2  a value substantially twice this. The resistor R 4  is included to prevent the circuit latching-up when T 1  goes into saturation. the amplifier is particularly useful for low voltage communication equipment, such as miniature radio receivers or hearing aids driven from small low voltage batteries. 
     Typical values of the components are: 
     R 1  =R 3  =120 ohms 
     R 2  =240 ohms 
     R 5  =10K ohms 
     R 4  =1K ohms 
     R 6  =10K ohms 
     R 7  =200K ohms 
     C=22 microfarads 
     The nominal loudspeaker impedance is typically 16 ohms. 
     The embodiment described was designed to operate with discrete silicon bipolar transistors and with a 1-2 volt supply. The invention also applies however to the use of germanium transistors, whereby a supply voltage of about 0.6 to 1.2 volts may be employed. The invention is applicable to circuits in integrated form as well as to discrete component circuits. 
     The circuit enables the output transistors T 3  and T 4  to have a stable and predictable bias current when quiescent. Also, the bias and drive arrangement provided by T 1  and T 2  with R 1 , R 2  and R 3  permits the output voltage at the collectors of T 3  and T 4  to swing the entire supply voltage less the saturation voltages of T 3  and T 4 . This is of especial advantage since normal complementary Class B push-pull amplifiers using silicon transistors do not permit operation at about 1 volt and allow large output voltage swings. 
     In a preferred mode of operation, the circuit is employed in a bridge configuration as shown in FIG. 2. Referring to the latter a pair of amplifier circuits as just described with reference to FIG. 1 are illustrated generally at 10 and 12 and with their corresponding input resistors R 6  and R 6  &#39;, respectively, and feedback resistors R 7  and R 7  &#39; respectively. Signal input is applied to R 6  at 14 and the output of the first amplifier 10 employed to drive the second amplifier 12 through R 6  &#39;. The gain of the second amplifier is set to unity and therefore, typically: 
     R 6  &#39;=R 7  &#39; 
     R 7  =50R 6   
     The bridge configuration is particularly useful for obtaining increased output powers at low voltage without the use of low impedance loads and large value low dissipation factor output capacitors. The elimination of the output capacitor gives both a cost and size factor saving. 
     A silicon integrated circuit implementation of the invention is shown in FIGS. 3 and 4. Referring to FIG. 3, transistors and resistors having the same functions (but not necessarily the same values) as the FIGS. 1 and 2 embodiment are similarly numbered. Thus, the biasing chain R 1 , T 1 , R 2 , T 2 , R 3  supplies push-pull output transistors T 3  and T 4 . The latter are in the form of current mirrors with T 6  and T 7 , respectively, to provide a defined current gain. A preferred mirror ratio T 6  :T 3  ; T 7  :T 4  is about 10:1. An additional drive stage T 8 , T 9  is employed to provide adequate overall current gain. The use of current mirrors plus an additional drive stage compensates for the somewhat inferior current gain performance of integrated pnp devices, particularly at high currents. 
     The resistors R 8 , R 9  allow effective gain boost at high drive levels whereas resistors R 10 , R 11  limit the drive current available to the current mirrors. 
     Because of the additional driver stage a phase reversal occurs which is recovered on feedback by the use of an additional phase inverter in the form of a further current mirror T 10 , T 11  typically with a ratio of about 1:5. 
     The amplifier A, within the dashed lines as shown may be used as a single-ended integrated amplifier with suitable A.C. coupling components to its input, A.C. compensation (&#34;comp&#34;), and output connections. Preferably however it is employed in a manner similar to that shown in FIG. 2--as a bridge amplifier in conjunction with a second, similar amplifier B, operated at unity gain and driven from the output of amplifier A. Such an arrangement is illustrated in FIG. 4. Both amplifiers A and B may be disposed on a single integrated circuit.