IMPROVEMENTS RELATING TO VARIABLE BANDWIDTH FILTERS

Abstract: A variable bandwidth filter comprises a transistor long-tailed pair. One of the transistors (T1) of the pair has a field effect transistor (Ft) which is connected in the collector circuit of the transistor and which is used as a variable collector load resistor. The field effect transistor is shunted by a capacitor (C1) and the gate of the field effect transistor is fed with a control voltage via a feed-back control loop arrangement which serves to maintain constant the d.c. voltage across the field effect transistor whereby the effective bandwidth of a filter circuit defined between the base of the said one transistor is arranged to be controllable in dependence upon the current in the common tail of the pair as determined by a variable current source (VCS) connected in the tail.

Description: IMPROVEMENTS RELATING TO VARIABLE BANDWIDTH FILTERS This invention relates to variable bandwidth filters, such as for use in pulse radar receivers. According to the present invention there is provided a variable bandwidth filter comprising a transistor long-tailed pair having in the collector circuit of one transistor of the pair a field effect transistor which is used as a variable collector load resistor and a capacitor or capacitive means arranged to shunt the field effect transistor, the gate of the field effect transistor being fed with a control voltage via a feed-back control loop arrangement which serves to maintain constant the d.c. voltage across the field effect transistor whereby the effective bandwidth of a filter circuit defined between the base of the one transistor of the pair and the collector circuit of that transistor is arranged to be controllable in dependence upon the current in the common tail of the pair as determined by a variable current source connected in the tail. By way of example one embodiment of the present invention will now be described with reference to the accompanying drawing which shows a circuit diagram of a multi-octave variable bandwidth filter. Referring to the drawing the variable bandwidth filter illustrated comprises a long-tailed pair of transistors Tl and T2 having a variable current source VCS connected in the common tail of the pair. The transistor Tl has connected in its collector circuit a field effect transistor FT having connected across it a capacitor Cl. The transistor FT serves effectively as a variable collector load resistor for the transistor Tl by maintaining the d.c. voltage across the transistor FT constant whilst a variable control voltage is applied to the gate of the field effect transistor from a feed back control loop arrangement FL including an amplifier AMP having a reference voltage Vref applied to one of its inputs and the other input being derived from the collector of the transistor Tl. The collector of transistor Tl is connected to the base of an output transistor OT the emitter of which is connected to an output terminal via a capacitor C2. Since the voltage drop across the transistor FT is maintained constant, variations in the collector current of the transistor Tl produced by varying the variable current source VCS will effectively produce changes in the resistance of the field effect transistor FT thereby varying the bandwidth of the filter defined by the transistor FT and the capacitor Cl in parallel therewith. Resistors Rl and R2 are provided, respectively, in the emitter circuits of the transistors Tl and T2. The values of resistors Rl and R2 are chosen so as to swamp the effect of any change in the conductance of the transistors Tl and T2 with temperature. The resistors Rl and R2 therefore provide gain stability of the circuit over a range of operating temperatures. The control range of the filter may be increased by substituting a variable capacitive diode for the capacitor Cl across the output load. This enables a 100:1 bandwidth ratio to be obtained with a 700 kHz minimum bandwidth. At the same time the filter shows good stability with changes in temperature.

Claims: CLAIHS : 1. A variable bandwidth filter comprising a transistor long-tailed pair having in the collector circuit of one transistor of the pair a field effect transistor which is used as a variable collector load resistor and capacitive means arranged to shunt the field effect transistor, the gate of the field effect transistor being fed with a control voltage via a feed-back control loop arrangement which serves to maintain constant the d.c. voltage across the field effect transistor whereby the effective bandwidth of a filter circuit defined between the base of the one transistor of the pair and the collector circuit of that transistor is arranged to the contollable in dependence upon the current in the common tail of the pair as determined by a variable current source connected in the tail. 2. A variable bandwidth filter as claimed in Claim 1, in which the collector of the said one transister of the long-tailed pair is connected to the base of an output transistor. 3. A variable bandwidth filter as claimed in Claim 1 or Claim 2, in which the feed-back control loop arrangement comprises resistor/capacitor means and an amplifier having a reference voltage applied to one of its inputs and the other input to the amplifier being derived from the collector of said one transistor of the long-tailed pair. 4. A variable bandwidth filter as claimed in Claim 1, in which the capacitive means comprises a single capacitor. 5. A variable bandwidth filter as claimed in Claim 1, in which the capacitive means comprises a variable capacitive diode. 6. A variable bandwidth filter as claimed in Claim 1, in which resistors are provided in the emitter circuits of the respective transistors of the long-tailed pair in order to swamp the effect of any change in the conductance of the transistors with temperature. C

Inventors: JARRETT ADRIAN EDWARD, JARRETT, ADRIAN, EDWARD
Applicants: PLESSEY OVERSEAS, PLESSEY OVERSEAS LIMITED
Application year: 1987
Main classification: H03H11
IPC codes: G11B27