GB2268649A - Reduction of parametrically-generated negative resistance in amplifier input using feedback - Google Patents

Abstract

This invention relates to the use of feedback circuitry in a high-efficiency power amplifier using gallium arsenide heterojunction bipolar transistors (GaAs HBTs). The feedback reduces the effects of parametrically- generated negative resistance at the amplifier input terminals. If uncompensated, this negative resistance often acts in conjunction with the remaining amplifier circuitry to generate spurious (undesired) signals. The spurious signals usually appear as oscillations at sub-multiples of the signal frequency, or as increased noise-spectral density over a band of frequencies. This enables HBTs to operate in high-efficiency amplifier circuits with minimum tendency to generate spurious outputs. <IMAGE>

Description

IMPROVEMENTS IN OR RELATING TO AMPLIFIERS This invention relates to improvements in or relating to amplifiers, in particular to power amplifiers.

The Gallium Arsenide heterojunction bipolar transistor (GaAs HBT), for example, is an attractive device for radio-frequency power-amplifier applications requiring high poweradded efficiency. High-efficiency amplifiers are essential to self-powered equipment such as battery-operated portable radio transmitters, for example. Power-added efficiency is a figure of merit which indicates the amount of DC input power required to produce a specified output-signal amplitude from a given input-signal amplitude. GaAs HBTs typically demonstrate, for example, greater than 70% power-added efficiency when operated at the common cellular-telephone frequencies near 900 MHz.

GaAs HBT fabrication processes are compatible with monolithic circuit-integration techniques; allowing resistors, capacitors, diodes, microstrip transmission lines, and other structures to be provided on the same substrate as the transistors. This integration capability is another asset when the size and weight restrictions of portable equipment are considered.

A high-efficiency power-amplifier design seeks to maximize the ratio of signal-power output to DC-power input. High-efficiency power amplifiers are usually operated at conduction angles of 180 degrees or less (Class-B or Class-C). The conduction angle refers to the portion of one period of a sinusoidal input signal over which the transistor is "on", or conducting. A full period of the input signal contains 360 degrees.

If an amplifier transistor is biased such that it is conducting with no input signal, then for very small signals the conduction angle will be 360 degrees. Because the collector current symmetrically increases and decreases during successive half-cycles of the input signal, the average collector current remains constant. This is known as Class-A operation.

As the amplitude of the input signal is increased, a point is reached where the negative excursions of collector current are equal in magnitude to the no-signal collector current.

Any further increase in input-signal amplitude forces the positive excursions of current to become larger than the negative excursions, which cannot become less than zero. The average collector current is then higher than the no-signal collector current. The conduction angle is between 360 and 180 degrees, and decreases with increasing inputsignal amplitude. This is known as Class-AB operation.

If the transistor is biased such that the no-signal collector current is nominally zero, but the transistor is on the verge of conducting, then one half-cycle of input signal will result in conduction and the other half-cycle will not. The conduction angle is 180 degrees.

This known as Class-B operation. Average collector current will increase as input-signal amplitude increases.

If the transistor is biased such that the the no-signal collector current is zero, and the current remains zero until the input-signal amplitude exceeds a finite threshold value, then the conduction angle is less than 180 degrees. This is known as Class-C operation.

Average collector current is zero when the input-signal amplitude is below the threshold.

Average collector current increases with increasing input-signal amplitude when the threshold is exceeded.

Of the operating modes described, Class-A operation normally results in the highest gain and lowest efficiency, while Class-C operation gives the lowest gain and highest efficiency. Class-AB and Class-B gain and efficiency characteristics are between these extremes.

High-efficiency power amplifiers are normally operated Class-B or Class-C. Under large-signal conditions, the base-emitter and base-collector junctions of the amplifier transistor experience large voltage excursions with each period of a sinusoidal input signal. These large voltage swings cause a nonlinear modulation of the junction capacitances. Much literature is devoted to the analysis and use of negative resistance generated by this capacitance modulation, commonly known as "pumping", in transistors and diodes. The negative resistance typically occurs at frequencies below the inputsignal frequency.

Intentional negative-resistance generation is utilized for signal amplification in a family of circuits generically known as parametric amplifiers. Intentional negative resistance is also used to create oscillators by incorporating the proper low-loss reactive circuits as frequency-determining elements. Oscillators are usually Class-A or Class-AB circuits, and employ negative-resistance techniques other than pumping.

In a high-efficiency power amplifier, the transistor is necessarily embedded in low-loss reactive circuits. These circuits often encourage spurious oscillations when the transistor is under large-signal Class-B or Class-C operation. Having the additional characteristic of high gain, the GaAs HBT is uniquely susceptible to oscillation and spurious-signal problems in high-efficiency Class-B and Class-C amplifier circuits.

Prior art has addressed the problem of oscillations and spurious signals in Class-B and Class-C amplifiers by increasing the loss of the reactive circuits external to the transistor.

The positive-real component of the lossy impedances is designed to cancel the negative resistance generated by the "pumped" junction capacitances, resulting in a stable and spurious-free amplifier. These techniques can be effective for spurious suppression but sacrifice performance.

One object of the present invention is to provide a high-efficiency amplifier free of oscillations and spurious signals having optimum performance.

Another object of the present invention is to provide a high-efficiency amplifier free from oscillations and spurious signals.

According to one aspect of the present invention, there is provided an amplifier for amplifying an input signal comprising a semiconductor device and a feedback loop, wherein the feedback loop is adapted to suppress negative resistance at the input of the amplifier.

The feedback reduces the effects of parametrically- generated negative resistance at the amplifier input terminals. If uncompensated, this negative resistance often acts in conjunction with the remaining amplifier circuitry to generate spurious (undesired) signals. These spurious signals usually appear as oscillations at sub-multiples of the signal frequency, or as increased noise-spectral density over a band of frequencies. This invention enables HBTs to operate in high-efficiency amplifier circuits with minimum tendency to generate spurious outputs.

Reference will now be made, by way of example, to the accompanying drawings, in which: Figure 1 is a schematic diagram of a power-amplifer transistor; Figure 2 is a block diagram illustrating one aspect of the present invention; and Figures 3 to 6 are circuit diagrams showing various embodiments of the present invention.

Referring to figure 1, a transistor is shown generally at 10. The transistor characteristics are shown on the diagram. Typically negative resistance may be produced at the base of the transistors Referring to the power amplifier circuit of figure 2, the negative resistance can be cancelled by using a feedback loop to simulate an impedance with a specified positive, real conductance or resistance. The feedback loop l2 is between the collector and the base of transistor 10. A positive conductance is simulated by the feedback network whenever an applied voltage results in a positive in-phase component of current being drawn by the feedback network from the signal source. A first matching circuit 14 is provided on the input of the transistor and a second matching circuit 16 is provided at the output of the transistor. In addition, the circuit may include a first bias circuit 18 connected between the base and a base voltage source Vb and a second bias circuit 20 connected between the collector and a collector voltage source Vc.

Optimum performance of high-efficiency amplifiers requires low-loss terminations at integral multiples (harmonics) of the signal frequency. For example, it has been demonstrated theoretically and experimentally that the second-harmonic voltage present at the transistor's base and collector terminals should be minimised. This requires a nearly-short-circuit terminating impedance. Because it is implemented directly between the base and collector terminals of transistor 10, the feedback does not interfere with the capability of the terminating networks 14 and 16 to present short circuit impedances at the second harmonic frequency. This preserves efficiency while ensuring spurious-free, stable operation of the amplifier.

In the simplest case, the signal voltages at the base and collector of amplifier-transistor 10 have a phase relationship of 180 degrees, and the feedback element 12 is a resistor.

The positive-real component of the feedback-simulated impedance is dependent on the resistor value, and on the magnitudes of the signal voltages at the base and collector.

In a first embodiment shown in Figure 3, amplifier transistor 10 is, for example, a GaAs HBT operated in Class-B or Class-C mode. Terminating networks 14 and 16 couple the signals in and out of the amplifier, as well as present the correct harmonic impedances for high-efficiency operation. Cancellation of the negative impedance (caused by nonlinear pumping at the signal frequency) is provided by feedback resistor 22. A capacitor 24 is also provided, this is optional. If desired, capacitor 24 can be used to prevent DC leakage between the collector and base terminals through resistor 22.

Parallel-resistive feedback from collector to base of the amplifier transistor may also have the sometimes-undesireable effect of reducing gain. This effect may be minimized by designing the feedback network to have frequency-dependent characteristics. These characteristics may be optimized such that the signal-frequency response of the amplifier is preserved or enhanced, while cancelling negative conductance at other frequencies.

A second embodiment, shown in Figure 4, addresses the problem of feedback-caused gain reduction at the signal frequency. The transistor 12, terminating networks 14 and 16, feedback resistor 22, and optional DC-blocking capacitor 24 are as described in the first embodiment. An inductor 26 and second capacitor 28 form a parallel-resonant network 30 in the feedback loop, the parallel-resonant network tuned to the signal frequency. When the circuit is resonant, no signal is coupled from the transistor collector to its base. This effectively removes the feedback at the resonant (signal) frequency, eliminating its effect on amplifier gain. At frequencies other than resonance, the signal is coupled through the feedback network and the function of negative-resistance cancellation is accomplished.

As the signal frequency becomes higher, realisation of the parallel-resonant circuit 30 becomes more difficult due to the need to control the series electrical-path length from collector to base. This generally requires small physical dimensions in the feedback network.

A third embodiment, shown in Figure 5, may be realized with a smaller input-to-output physical length than the second embodiment. This is due to the fact that the resonantcircuit elements are no longer in series with the feedback resistor. The feedback resistor is split into two elements, 22a and 22b. At the common node of the resistors, frequencydependent feedback elements 26 and 28 are now arranged to form a series-resonant circuit connected to ground. The circuit is resonant at the signal frequency, creating a short-circuit to ground and preventing a signal from being coupled from the collector to the base. Resistors 22a and 22b prevent this signal-frequency short circuit from being applied to either the collector or base terminals of amplifier transistor. Again, capacitor 24 is optional.

A fourth embodiment, shown in Figure 6, is a variation of the third embodiment. The series-resonant circuit is implemented using an open-circuited transmission line 32. This element simulates the properties of series resonance when its electrical length is 90 degrees. The physical length of the transmission line is designed such that the electrical length becomes 90 degrees at the signal frequency. As in the third embodiment, a signalfrequency short-circuit to ground is created at the common node of resistors 22a and 22b.

This prevents a signal from being coupled from collector to base of the amplifier transistor, thereof preserving signal-frequency gain. This transmission-line implementation of the frequency-dependent feedback is useful at frequencies where realization of inductive/capacitive resonant circuits might be difficult.

The resonant circuit may be located on the line between the emitter and ground in an alternative embodiment of the invention. This will have effectively the same function as the above described resonant circuits.

For the discussions above, the power-amplifier transistor is assumed to be NPN-type.

For normal operation, this implies positive base-emitter voltage, positive current into the base terminal, positive collector-emitter voltage, and positive current into the collector terminal. For PNP-type transistors, the conventions would be reversed.

It will be appreciated that this invention is ideal for use in a telephone, such as a cellphone. However, other applications are equally as useful, for example, in pagers.

Claims (12)

1. An amplifier for amplifying an input signal comprising a semiconductor device and a feedback loop, wherein the feedback loop is adapted to suppress negative resistance at the input of the amplifier.

2. An amplifier according to Claim 1, wherein the feedback loop is frequency dependant and suppress negative resistance as a function of the signal freqiiency.

3. An amplifier according to Claim 1 or Claim 2, wherein the feedback loop comprises a resonant circuit which is tuned to suppress feedback at the resonant frequency of the input signal.

4. An amplifier according to Claim 3, wherein the resonant circuit is a parallel resonant circuit.

5. An amplifier according to Claim 3 or Claim 4, wherein the resonant circuit comprises a resistive element and an inductive element.

6. An amplifier according to any preceding claim, wherein the feedback loop includes a resistive element.

7. An amplifier according to any preceding claim, wherein the feedback loop includes a capacitive element.

8. An amplifier according to any preceding claim, wherein the feedback loop includes an inductive element.

9. An amplifier according to any preceding claim, wherein the transistor is a heterojunction bipolar transistor.

10. An amplifier according to any preceding claim, wherein the transistor is fabricated of Gallium Arsenide (GaAs).

11. An integrated circuit including at least one amplifier according to any preceding claim.

12. A cell phone including at least one amplifier according to any of claims 1 to 10.