Amplifiers

Abstract: An amplifier comprising an emitter-coupled pair of transistors (1,2) having inductive load impedances (11,12), with capacitive feedback elements (15,16) tuned with the inductive loads and the capacitive input (C IN ) of the following stage to be resonant at the range of frequencies to be amplified. The reactive loads and feedback elements introduce less noise than with resistive components.

Description: The present invention relates to amplifiers, and in particular although not exclusively to low-noise radio frequency amplifiers. The performance of a radio frequency receiver is limited by two conflicting requirements:- 1) The noise level generated in the receiver itself, which dictates the minimum received signal level that will meet a given signal to noise ratio. 2) The ability of the receiver to receive weak signals in the presence of much larger signals at frequencies close to that of the weak signals. Adjacent channel rejection is conventionally achieved in the IF filters of a receiver, so that any front-end amplifying stage, prior to the first frequency changer, may have to cope with large adjacent-channel signals and at the same time introduce the minimum possible level of noise. If such front-end amplifying stages were perfectly linear they would be able to cope with any level of adjacent-channel signal. However, in practice, such stages are not perfectly linear, and can give rise to intermodulation products that may fall within the wanted channel, contributing to the noise and interfering with the reception of weak signals in the wanted channel. Such amplifier stages are also limited by the maximum signal level that can be handled without compression or clipping occurring. Conventional methods of improving the linearity of front-end amplifying stages involve the use of series and shunt feedback together with as much open-loop gain as can be generated without exceeding acceptable limits of stability and while retaining a suitable time domain response. The feedback is usually effected by way of resistors, which add noise to the amplifier and result in a degraded noise figure. The effect can be minimised by using small values of resistance, but this then requires high power levels if the advantages of linearity and large signal handling are to be retained. According to one aspect of the present invention a low noise transistor amplifier stage comprises a junction transistor having inductive impedance means in its emitter circuit, inductive collector load means, and a capacitor connected in a feedback path from its collector electrode to its base electrode, the values of the inductive impedance means and the feedback capacitor being such that the phase shifts introduced thereby substantially cancel one another at the range of frequencies to be amplified, and the value of capacitance at the output of said amplifier stage, including that presented by said feedback capacitor, being substantially resonant with said inductive collector load means at the range of frequencies to be amplified. According to another aspect of the present invention a low noise differential input/differential output amplifier stage comprises a pair of junction transistors having their emitter electrodes connected to a common current source by way of respective inductive impedance means and their collector electrodes connected to respective inductive load means, and respective feedback capacitors connecting the collector electrodes of said transistors to their respective base electrodes, the values of the inductive impedance means and the feedback capacitors being such that the phase shifts introduced thereby substantially cancel one another at the range of frequencies to be amplified, and the values of capacitance at the outputs of said amplifier stage, including that presented by said feedback capacitors, being arranged to be substantially resonant with said inductive load means at the range of frequencies to be amplified. A low-noise radio frequency amplifier stage in accordance with the present invention will now be described by way of example with reference to the accompanying drawing, which shows the amplifier stage diagrammatically. Referring to the drawing, the amplifier stage comprises a pair of NPN junction transistors 1 and 2 the emitter electrodes of which are connected by way of respective inductive elements 3 and 4 to a common current source comprising a transistor 5, resistor 6 and capacitor 7, the value of current provided by said source being dependent upon the value of a bias signal applied to a terminal 8. Radio frequency input signals are applied by way of input terminals 9 and 10 to the base electrodes of the transistors 1 and 2, and output signals developed across inductive collector load impedances 11 and 12 are applied to a following stage 13 represented as a capacitive input impedance C IN and a current source 14. Capacitors 15 and 16 provide feedback from the collector circuits of the transistors 1 and 2 to their base circuits. The use of reactive components rather than resistive components to provide the feedback introduces much less noise and consequently little increase in the noise figure over that associated with the transistors themselves. The values of the reactive components are chosen so that the capacitances at the collector electrodes of the transistors 1 and 2, including the capacitance values presented by the capacitors 15 and 16, the collector capacitances of the transistors 1 and 2 and the input capacitance C IN of the following stage 13, are arranged to be resonant with the inductive load impedances 11 and 12 at the range of frequencies to be amplified. The phase shifts introduced by the capacitors 15 and 16 and the inductive elements 3 and 4 are arranged to cancel, so that the input impedance presented at the terminals 9 and 10 is almost entirely resistive, and it is possible to define a low standing wave ratio real input impedance. The dominant parasitics of the transistors 1 and 2, the collector-base capacitances, are effectively in parallel with the respective capacitors 15 and 16 and are tuned out. At the same time the tuning of the output circuit with the capacitive input impedance of the following stage 13 achieves optimum power matching. The circuit therefore enables accurate input and output power matching, low noise figure and large signal handing with minimum power consumption.

Claims: A low noise transistor amplifier stage comprising a junction transistor (1) having inductive impedance means (3) in its emitter circuit, inductive collector load means (11), and a capacitor (15) connected in a feedback path from its collector electrode to its base electrode, the values of the inductive impedance means (3) and the feedback capacitor (15) being such that the phase shifts introduced thereby substantially cancel one another at the range of frequencies to be amplified, and the value of capacitance at the output of said amplifier stage, including that presented by said feedback capacitor (15), being substantially resonant with said inductive collector load means (11) at the range of frequencies to be amplified. A low noise differential input/differential output amplifier stage comprising a pair of junction transistors (1, 2) having their emitter electrodes connected to a common current source (5, 6, 7, 8) by way of respective inductive impedance means (3, 4) and their collector electrodes connected to respective inductive load means (11, 12), and respective feedback capacitors (15, 16) connecting the collector electrodes of said transistors (1, 2) to their respective base electrodes, the values of the inductive impedance means (3, 4) and the feedback capacitors (15, 16) being such that the phase shifts introduced thereby substantially cancel one another at the range of frequencies to be amplified, and the values of capacitance at the outputs of said amplifier stage, including that presented by said feedback capacitors (15, 16), being arranged to be substantially resonant with said inductive load means (11, 12) at the range of frequencies to be amplified.

Inventors: SAWYER DAVID ALBERT, SAWYER, DAVID ALBERT
Applicants: MITEL SEMICONDUCTOR LTD, MITEL SEMICONDUCTOR LIMITED
Application year: 1993
Main classification: H03F1
IPC codes: H03F3, H03F1