Patent GB-1324281-A (GB)
Title: Transmission gate including a transistor and biasing circuits
IPC: G04G99/00
Filed: 1970-08-26
Abstract: 1324281 IGFET shift register RCA CORPORATION 26 Aug 1970 [4 Sept 1969] 41085/70 Heading G4C [Also in Division H3] In a circuit having a transmission gate including the conduction path of a transistor 32, Fig. 2A, for coupling an input signal at 12 to the first electrode of an amplifier 27a, the transistor 32 exhibits a threshold voltage and consequently offsets, with respect to a reference potential, the input signal transmitted to the first electrode by an amount substantially equal to the threshold voltage of the transistor 32 when the direction of conduction in its transmission path causes the transistor 32 to operate in the follower mode and a biasing circuit 50 is coupled to a second electrode of the amplifier 27a for providing a bias voltage substantially equal in magnitude to the threshold voltage and poled in a direction to substantially eliminate the potential difference across the first and second electrodes due to the threshold voltage. With the input at 12 at +V and O applied to gate 38 transistor 32 conducts and the load which includes distributed or discrete capacitor 28 charges so that 24 becomes + V and transistor 27a turns on to make output terminal 30 clamped to the voltage across the bias circuit 50. When the input 12 is O and transistor 32 is enabled by a O gate voltage at 38 the transistor 32 operates in the follower mode and the voltage across the capacitor 28 discharges until at the threshold voltage of the transistor 32. As the voltage drop across the transistor 52 is arranged to equal this threshold voltage the gate to source potential of transistor 27a is zero and transistor 27a is cut off. The output at 30 rises to + V as determined by load 58 which may be resistive or an active device such as a transistor 27b, Fig. 3. The gate transistor 32 may be of the opposite conductivity type (42, Fig. 2B, not shown) and the biasing device 50 is then (60) connected beween the load transistor 27b and + V . Temperature tracking is obtained by forming the transistor 52 of the bias circuit 50 by the same process and in the same manner as transistor 32 so that their threshold voltages are substantially equal. The transmission gate transistors 32 can be used to form a shift register, Fig. 3, each stage 120 having gates 32a, 32b controlled in antiphase by two clock pulse sources # 1 , # 2 . All of the stages have a common bias circuit 50 for compensating for the voltage offsets in the gates 32. A complementary shift register arrangement may be obtained using the gates of (Fig. 2B, not shown). Alternatively the shift register may include alternate stages of opposite conductivity type (Fig. 4, not shown), so that only a single phase clock is required. The biasing circuit 50 may instead of the transistor 52, use a biasing means such as Zener diodes or resistive voltage dividers. A circuit for restoring the full input signal voltage swing from the output of the last stage of the shift register of Fig. 3 is disclosed (Fig. 5). A low level output from the transmission gate transistor 32 in the last stage (Nth stage) at 24b turns transistors Qnb and Qna on and off. Gate transistor 106 is turned on and together with transistors 108, 110 forms a potential divider biasing output transistor 104a on. Transistor 104b is off so that the output at 112 is O. When high level output occurs at 24b transistors Qnb and Qna are biased off and on and transistor 104b is forward biased causing the output at 112 to start to rise to + V. Transistor 106 acts as a source follower and conducts to discharge the potential on the gate of the transistor 104a. When the potential on the gate of the transistor 104a reaches the threshold value of gate transistor 106 this transistor 106 cuts off thereby allowing transistors 108, 110 to clamp the gate of transistor 104a to O and turn it off. The transistors used may be IGFET of the enhancement and depletion types, or bipolar transistors or junction FET.