Patent GB-889017-A (GB)
Title: Improvements in transistor circuits
Inventor: ROZNER FELIX
IPC: H04N5/067
Filed: 1957-05-22
Abstract: 889,017. Transistor amplifying circuits. FERGUSON RADIO CORPORATION Ltd. May 14, 1958 [May 22, 1957], No. 16318/57. Class 40(6). [Also in Group XL(b)] A transistor circuit for effecting simultaneously the combination and D.C. restoration of a plurality of input signals (e.g. synchronizing, blanking and television picture signals) comprises a corresponding number of transistors having the respective input signals applied through capacitors to their bases and an output load arranged so as to be common to the collector-emitter paths of all the transistors and connected in the emitter lead of at least one of the transistors. Thus Fig. 3 shows a circuit comprising three alternative inputs 26, 27, 28 such that the output developed at terminal 24 due to an input at terminal 26 does not exceed voltage level E1, that developed at terminal 24 due to an input at terminal- 27 lies between levels El and E2, E2 being more positive than El while the output developed due to an input at terminal 28 lies between levels E3 and E2, E3 being more positive than E2. Signals applied to the base of transistor 21 cannot become more positive than voltage level El owing to conduction in the collector-base diode of the transistor 21, and corresponding output signals are consequently restored to that level. Similarly input signals at the base of transistor 22 cannot exceed a voltage level E2 but similarly cannot fall below voltage level El since to do so would cause the base-emitter diode of transistor 21 to conduct and hold the corresponding output signal at that level. The output signal due to an input to terminal 28 similarly is held between levels E2 and E3. A modified form of circuit, Fig. 5 (not shown), in which transistor 23 is replaced by a PNP transistor having its emitter electrode connected to the positive terminal of the bias source is used for mixing synchronizing pulses, blanking pulses and blanked picture signals in a television system, each signal being held within its own proper range of levels. A simplified form of the circuit of Fig. 5 is shown in Fig. 4 and comprises transistors 29, 33 of opposite conductivity types the former operating as an emitter-follower and the latter operating as a grounded-emitter amplifier. When an input is applied to terminal 30 the output at terminal 31 cannot exceed voltage level El, while when an input is applied to terminal 34 the output at terminal 31 does not fall below the voltage level El. Fig. 2 shows a circuit not falling within the scope of the invention in which input signals are applied from a terminal 20 to an emitter-follower amplifier 16, the base electrode of a futher transistor 15 connected in parallel with the transistor 16 being maintained at a steady bias voltage level El so that should the potential at the output terminal 18 tend to fall below the level El, the emitter-base diode of the transistor 15 conducts to hold the potential at the terminal 18 at that level. For high-frequency operation an impedance incorporating a capacitive reactance may be placed in series with the base electrode of the transistor 15 to compensate for the inductive reactance exhibited by the transistor. Specification 733,294 is referred to.