Abstract:
A high-speed cascode amplifier includes a cascode input stage. The cascode input stage sinks a cascode current in proportion to an input signal. The cascode current is sourced through two conduction paths. A first conduction path corresponds to the operating current for a diode multiplier transistor. The second conduction path corresponds to an alternate conduction path other than through the diode multiplier transistor. The size of the diode multiplier transistor is reduced by proper arrangement of the alternate path, such that the frequency response of the amplifier is improved. The V CE  of the diode multiplier transistor is maintained at a stable level by adjusting the bias current of the diode multiplier transistor. The diode multiplier provides a temperature-compensated bias current to the class AB amplifier stage for producing the output signal.

Description:
FIELD OF THE INVENTION 
     The present invention relates to a method and apparatus for biasing high-speed class AB amplifiers. More specifically, the present invention is directed to improving the frequency response of a class AB amplifier stage by properly arranging a diode multiplier circuit that is in the bias stage of the amplifier. 
     BACKGROUND OF THE INVENTION 
     A diode multiplier includes a transistor and a resistor network. The transistor has an inherent base-emitter voltage (V BE ). The resistor network in the diode multiplier is arranged such that an output voltage that corresponds to a multiplication of the base-emitter voltage is provided. The diode multiplier ideally maintains an output voltage that is constant. In practice, the output voltage increases as the collector current of the transistor in the diode multiplier increases. 
     A conventional diode multiplier may be arranged in a cascode amplifier. As the input signal to the amplifier increases, the collector current of the transistor in the diode multiplier increases. A large transistor, or a Darlington device, can be used to minimize the increase in the collector current of the transistor in the diode multiplier. The use of such large devices results in a corresponding increase in the parasitic collector capacitance of the device in the diode multiplier. The increase of the capacitance causes an increase in the RC time constant (formed by a load resistance and the collector capacitance, among others). Conventional diode multipliers have poor frequency response characteristics due to the collector capacitance. 
     The transistor in the diode multiplier is sized to bias the complementary inputs of a class AB amplifier. The bias current provided for the class A stage of the class AB amplifier must be high when the amplifier is operating near the supply rail. However, the bias currents should be limited to minimize excessive bias currents at low voltage outputs of the amplifier. Excessive bias currents for low voltage outputs of the amplifier can result in “shoot through” of the class B stage of the class AB amplifier. “Shoot through” may result in excessive power dissipation and possibly thermal runaway. The bias current provided by the diode multiplier must also be sufficiently low for when the amplifier is operating near the supply rail, but not so low as to provide insufficient bias current at low voltage outputs of the amplifier. An insufficient bias current at low voltage can result in “crossover distortion” of the class A stage of the class AB amplifier. 
     SUMMARY OF THE INVENTION 
     The present invention relates to a method and apparatus for biasing high-speed class AB amplifiers. More specifically, the present invention is directed to improving the frequency response and reducing the power consumption of a class AB amplifier stage by arranging a diode multiplier circuit in the bias stage of the amplifier. 
     A high-speed amplifier according to the present invention includes a cascode input stage for receiving an input voltage, a diode multiplier for providing a temperature-compensated bias current, and a class AB amplifier stage for providing an output signal. The cascode input stage sinks a cascode current in proportion to an input signal. The cascode current is sourced in part through a first load resistor that limits a current through the transistor of the diode multiplier. The cascode current is also sourced in part through a second load resistor that sources a majority of the cascode current. Accordingly, the diode multiplier transistor may be made smaller than the cascode transistor of the cascode input stage because the transistor of the diode multiplier carries less current than a transistor in the cascode input stage. The reduction in size of the diode multiplier transistor enhances the frequency response of the diode multiplier. Any rise in voltage in the diode multiplier transistor V CE  is applied to a resistor network that increases the bias current of the diode multiplier transistor, which negates the rise of the diode multiplier transistor V CE  and maintains the V CE  at a stable level. The diode multiplier provides a temperature-compensated bias current to the class AB amplifier stage for producing the output signal. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a schematic diagram of an example class AB amplifier that employs a diode multiplier in accordance with the present invention. 
     FIG. 2 is a graph of the collector-to-emitter voltage of a diode multiplier transistor that is in accordance with the present invention. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     Throughout the specification, and in the claims, the term “connected” means a direct electrical connection between the things that are connected, without any intermediary devices. The term “coupled” means either a direct electrical connection between the things that are connected, or an indirect connection through one or more passive or active intermediary devices. The term “circuit” means one or more passive and/or active components that are arranged to cooperate with one another to provide a desired function. The term “signal” means at least one current signal, voltage signal, electromagnetic wave signal, or data signal. The meaning of “a”, “an”, and “the” include plural references. The meaning of “in” includes “in” and “on.” The term “operating current” means the current that flows in a device. 
     The present invention is directed to a method and apparatus that improves the frequency response and reduces the power consumption in a class AB amplifier. A diode multiplier circuit is employed in a bias stage for the amplifier. The diode multiplier circuit includes a diode multiplier transistor. A parasitic capacitance is present at the node that includes the collector of the diode multiplier transistor. The parasitic capacitance is a lumped capacitance that is primarily due to the parasitic capacitance from the collector of the diode multiplier transistor. Other capacitances from other devices and structures present at the diode multiplier transistor collector node also contribute to the parasitic capacitance. The capacitance present at the diode multiplier transistor collector node is reduced by reducing the size of the collector of the diode multiplier transistor. The frequency response of the amplifier is increased by reducing the parasitic capacitance at the diode multiplier transistor collector node. 
     A second load resistor is arranged to reduce the operating current (i.e. collector current) of the diode multiplier transistor. The diode multiplier transistor may be reduced in size by reducing the current-carrying requirements of the diode multiplier transistor. The frequency response of the diode multiplier transistor is increased due to the reduced parasitic capacitance that results from using a smaller transistor. The second load resistor is also arranged to send an additional base current to the base of the diode multiplier transistor in response to an increase in the collector-emitter voltage (V CE ) of the diode multiplier transistor. The base current is proportional to the operating current of the diode multiplier transistor and effectively prevents the V CE  of the diode multiplier transistor from increasing when the current flow through the diode multiplier transistor increases. Accordingly, the base current is increased whenever the V CE  of the diode multiplier transistor increases and acts to reduce the increase of the V CE . 
     FIG. 1 is a schematic diagram of an example class AB amplifier ( 100 ) that employs a diode multiplier circuit that is in accordance with the present invention. The class AB amplifier ( 100 ) includes seven transistors (Q 1 -Q 7 ),  12  resistors (R 1 -R 12 ), and two capacitors (C 1 -C 2 ). V CC  and V BB  are provided as voltage supplies for the amplifier. In another example embodiment, transistors that are of a type that is complementary to the type of each transistor that is described herein may also be used in accordance with the instant invention. Using complementary types of transistors to those described herein may be accomplished by using voltage supplies that have an opposite polarity to those described. 
     Transistor Q 1  is an NPN transistor that has a collector coupled to node N 1 , a base coupled to node N 2 , an emitter coupled to node N 4 , and a substrate coupled to ground. Transistor Q 2  is an NPN transistor that has a collector coupled to node N 4 , a base coupled to V BB , an emitter coupled to the collector of transistor Q 3 , and a substrate coupled to ground. Transistor Q 3  has a collector connected to the emitter of transistor Q 2 , a base connected to node IN, an emitter coupled to node N 5 , and a substrate coupled to ground. Transistor Q 4  is an NPN transistor that has a collector coupled to V CC , a base coupled to node N 1 , an emitter coupled to node N 7 , and a substrate coupled to ground. Transistor Q 5  is a PNP transistor that has an emitter coupled to node N 9 , a base that is coupled to node N 4 , a collector that is coupled to ground, and a substrate that is coupled to node OUT. Transistor Q 6  is an NPN transistor that has a collector coupled to V CC , a base that is coupled to node N 7 , an emitter that is coupled to resistor R 10 , and a substrate that is coupled to ground. Transistor Q 7  is a PNP transistor that has an emitter coupled to resistor R 11 , a base that is coupled to node N 9 , a collector that is coupled to ground, and a substrate that is coupled to node OUT. 
     Resistor R 1  is coupled between V CC  and node N 1 . Resistor R 2  is coupled between V CC  and node N 3 . Resistor R 3  is coupled between node N 1  and node N 2 . Resistor R 4  is coupled between node N 2  and node N 3 . Resistor R 5  is coupled between node N 3  and node N 4 . Resistor R 6  is coupled between node N 5  and ground. Resistor R 7  is coupled between node N 7  and node N 8 . Resistor R 8  is coupled between node N 8  and node N 9 . Resistor R 9  is coupled between node N 8  and node OUT. Resistor R 10  is coupled between the emitter of transistor Q 6  and node OUT. Resistor R 11  is coupled between the emitter of transistor Q 7  and node OUT. Resistor R 12  is coupled between node OUT and capacitor C 2 . Capacitor C 1  is coupled between node N 5  and node OUT. Capacitor C 2  is coupled between resistor R 12  and ground. 
     The amplifier of FIG. 1 includes a diode multiplier circuit (comprising transistor Q 1 , load resistors R 1  and R 2 , and a resistor network that contains resistors R 3 -R 5 ), a cascode stage (comprising transistors Q 2  and Q 3  and emitter resistor R 6 ), and a class AB output stage (comprising transistors Q 4 -Q 7 , resistors R 7 -R 12 , and capacitor C 1  and C 2 ). The association of components with a named portion of the circuit does not limit the operation or association of the components to a named portion of the circuit. The association of the components with a named portion of the circuit is given for the sole purpose of convenient reference in this discussion. 
     The cascode input stage sinks a cascode operating current in proportion to an input signal. The cascode operating current is sourced in part by the diode multiplier transistor collector current. The cascode operating current is limited by a first load resistor (R 1 ). The cascode operating current is also sourced by a bypass current (that does not flow through the diode multiplier transistor collector), which is limited by a second load resistor R 2 . The second load resistor (R 2 ) is thus arranged to increase the cascode operating current beyond the collector current capabilities of the diode multiplier transistor. The gain of the amplifier is defined by the ratio of the emitter impedance of the lower cascode transistor (including resistor R 6 ) to the collector impedance of the upper cascode (including the first and second load resistors R 1  and R 2  of the diode multiplier circuit). 
     The diode multiplier circuit is arranged to maintain a relatively constant voltage (i.e. the V CE  of the diode multiplier transistor Q 1 ) between node  1  and node  4 , and to provide a temperature compensated bias current for the output stage. FIG. 2 is a graph of the collector-to-emitter voltage of the diode multiplier transistor Q 1  that is in accordance with the present invention. The V CE  of the diode multiplier transistor Q 1  is maintained within a predetermined range that is within approximately 25 percent of the voltage operating range (at a constant operating temperature). The start of the operating range corresponds to a point of the V CE  curve that has a tangent of 45 degrees for a given operating temperature. In an example embodiment, the V CE  of diode multiplier transistor Q 1  reaches a voltage of 1.2 V when the output voltage is around 16 V at a temperature of zero degrees Celsius. 
     The class AB output stage is arranged to provide an output signal in response to the cascode operating current. Transistors Q 4  and Q 5  are arranged to provide class A amplification, while transistors Q 6  and Q 7  are arranged to provide class B amplification. The amplifier ( 100 ) may be implemented in an integrated circuit, although other implementations are possible. 
     The amplifier of FIG. 1 is a high-speed cascode class AB amplifier that is capable of driving a highly capacitive or resistive load. V CC  provides a supply voltage in a range of 65 V-85 V, while V BB  provides a bias voltage in a range from 8 V-12 V. An analog input signal is provided in the range of 0.5 V-3 V is applied to the input (node IN) of the amplifier. The amplifier amplifies the input signal and provides an output signal at the output (node OUT). The output signal is capable of driving resistive loads or highly capacitive loads such as the cathode of a CRT. 
     In operation, an increase in the input signal (applied at node IN) causes the collector current of transistor Q 3  to increase. The increase in the collector current of transistor Q 3  causes the collector current of cascode transistor Q 2  to increase. The increase in the collector current of cascode transistor Q 2  causes the collector current of the diode multiplier transistor Q 1  to increase. As the collector current of the diode multiplier transistor Q 1  increases, the V CE  of transistor Q 1  also tends to increase. The increase in the V CE  of transistor Q 1  causes the bypass current flow through resistors R 2  and R 5  to increase. As the voltage at node N 4  decreases, the voltage drop across resistor R 5  increases. The voltage drop across resistor R 5  causes a rise in the voltage across resistor R 4 , which results an increase in the base current to transistor Q 1 . The additional base current compensates for the higher collector current such that a stable V CE  voltage is maintained. 
     Similarly, a decrease in the input signal (applied at node IN) causes the collector current of transistor Q 3  to decrease. The decrease of the collector current in transistor Q 3  causes the collector current of cascode transistor Q 2  to decrease. The decrease in the collector current of cascode transistor Q 2  causes the collector current of the diode multiplier transistor Q 1  to decrease. As the voltage at node N 4  increases, the voltage drop across resistor R 5  decreases. The voltage drop across resistor R 5  causes a decline in the voltage across resistor R 4 , which results in a lower base current to transistor Q 1 . The decrease in the base current compensates for the lower collector current such that a stable V BE  voltage is maintained. 
     The diode multiplier circuit is arranged to provide a bias voltage to bias the class A transistors (Q 4  and Q 5 ). The diode multiplier circuit, which is formed by resistor R 3 , transistor Q 1 , and resistor R 4 , roughly provide a bias voltage that is equivalent to the voltage drop of three series connected diodes. 
     The diode multiplier circuit forms a temperature-compensated bias current when the diode multiplier circuit is located on the same substrate as transistors Q 4  and Q 5 , which are driven by the diode multiplier circuit. Transistors Q 1 , Q 4 , and Q 5  are at approximately the same temperatures when they are thermally coupled together. Thermal coupling may be accomplished, for example, by providing the transistors on the same substrate, or by collocating discrete transistors in similar thermal environments. Accordingly, the relative electrical performance of the devices (Q 1 , Q 4 , and Q 5 ) track one another across the temperature operating range for those transistors. Temperature compensation allows for wider range of operation while avoiding undesirable crossover distortion or “shoot through.” 
     Transistors Q 4  and Q 5  are arranged as pre-driver emitter followers that perform class A amplification in the class AB stage of the amplifier ( 100 ). Accordingly, transistors Q 4  and Q 5  maintain a quiescent bias current for all phases of the applied input signal and thus avoid undesirable crossover distortion. The quiescent current is determined by an adjusted bias voltage (the V CE  of transistor Q 1  minus the V BE S of transistors Q 4  and Q 5 ) divided by the sum of the resistances for emitter resistors R 7  and R 8 . Thus, the output current of the amplifier is drawn through resistors R 7  and R 9  (or resistors R 8  and R 9 ). 
     Transistors Q 6  and Q 7  are each arranged in a Darlington configuration, where transistors Q 4  and Q 6  provide a NPN-type Darlington pair and transistors Q 5  and Q 7  provide a PNP-type Darlington pair. Transistors Q 6  and Q 7  perform class B amplification in the class AB stage of the amplifier ( 100 ). Accordingly, transistors Q 6  and Q 7  are active for less than half of the phases of the applied input signal and thus avoid undesirable “shoot through.” Additional output current of the amplifier ( 100 ) is drawn through resistor R 10  (or resistor Ri  1 ) when the output stage of the amplifier is in the class AB operation. Resistor R 9  is arranged to set the voltage of the output signal when transistors Q 6  and Q 7  are turned off. Resistor RIO and resistor R 11  limit the output current of transistors Q 6  and Q 7  respectively and provide for a balanced output. 
     Capacitor C 1  is a compensation capacitor, which provides feedback to the input stage for increasing the stability of the amplifier at high frequencies. Resistor R 12  and capacitor C 12  depict a representative load of the amplifier. 
     Although the above-described circuit is arranged to bias a class AB amplifier, the diode multiplier circuit may be used in other electronic circuits. In one example, the diode multiplier circuit is used with a class A amplifier. In another example, the diode multiplier circuit is used with a push-pull output driver. In yet another example, the diode multiplier circuit is used with a class B amplifier. The diode multiplier circuit may be employed to bias any circuit that requires a relatively constant bias voltage. When employed in an integrated circuit, the diode multiplier circuit also provides compensation for temperature-based variations in the bias voltage requirements for various transistors. Although the diode multiplier circuit is described with a single transistor, other implementations may include more than a single transistor that are arranged provide a similar function. The above specification, examples and data provide a complete description of the manufacture and use of the composition of the invention. Since many embodiments of the invention can be made without departing from the spirit and scope of the invention, the invention resides in the claims hereinafter appended.