Patent GB-1263994-A (GB)
Title: Input circuit arrangements for transistors
IPC: H03F3/191
Filed: 1969-03-06
Abstract: 1,263,994. Transistor amplifying circuits; waveband switching. PHILIPS ELECTRONIC & ASSOCIATED INDUSTRIES Ltd. 6 March, 1969 [9 March, 1968], No. 11957/69. Headings H3Q and H3T. In an input circuit arrangement a signal is applied via input terminals 2, 3, a coupling network having a series inductor La, a parallel resonant circuit Ct, Cv, Co, Lp tuned by at least one variable capacity diode Cv and including at least one element capable of dissipating a portion of the power of the signal and an impedance inverting transformation network having a series inductor Li to the input of a common base transistor T, the coupling network, the inverting transformation network and the element or elements capable of dissipating signal power are proportioned such that the input impedance of the circuit substantially matches the impedance of the source of input signal and also provide a substantial optimum noise level in the transistor. The element or elements capable of dissipating signal power are at least partially formed by the natural losses of the variablecapacity diode Cv which may be represented as a resistor in series with Cv. The series inductors La, Li transform the resistance of the signal source to the input resistance of the transistor T to produce across the resonant circuit conductances which are inversely proportional to the square of the frequency. At least one additional loss resistor R I , R III  may be provided which may be transformed across the resonant circuit to produce a conductance (Gp, Fig. 2, not shown), which is inversely proportional to the square of the frequency. Loss resistors may be provided in series with Lp or Cv branches or in series with the parallel circuit of Lp and Cv. In the television tuner shown, switch S I , S III  provides variable capacity diode Cv I  with a reverse bias-tuning voltage for Band I reception whereas variable capacity diode Cv III  is forward biased so as to short out the Band III tuned circuit Lp III , Ct III , Co III , Cv III , R III . The opposite occurs for the other position of band switch S I , S III  where transistor T receives Band III signals as tuned by Cv III . The input to the Band III tuned circuit is via a HPF8 which passes signals of Band III and above. The input to both Band I and Band III circuits is via a HPF 7 passing signals of Band I and above. In an alternative input circuit (Fig. 4, not shown), the inductors La I  and La III  are fed from input terminal 2 via separate circuits each including a capacitor and an inductor tuned to the respective Bands I and III. In a single tuned circuit arrangement (Fig. 1, not shown), transformer coupling may be provided between input terminals 2, 3 and inductor La, between La and the parallel tuned circuit, between the tuned circuit and inductor Li and between Li and transistor T. The dispersion inductance of these transformers can form at least part of the inductors La, Li or Lp and the series inductors La or Li or the variable capacity diode may be connected to a tap of the inductor Lp. Specification 1,099,890 is referred to.