Abstract:
The present invention relates to an input buffer for a switched emitter follower-like track-and-hold amplifier comprising an input stage with an input transistor (Q 1 ), a first diode (Q 2 , Q 2   b ), a cathode side of which first diode (Q 2 , Q 2   b ) is connected to an emitter of the input transistor (Q 1 ), a first current source ( 4 ) between on the one hand the junction between the cathode of the first diode (Q 2 , Q 2   b ) and the emitter of the input transistor (Q 1 ) and on the other hand a first supply voltage line ( 2 ), an anode of the first diode (Q 2 , Q 2   b ) being connected to a track-and-hold controlled emitter follower ( 6 ). The first current source ( 4 ) is a non-switched constant current source, and a second current source (M 1 ) is connected between a collector of the input transistor (Q 1 ) and a second supply voltage ( 3 ). A second transistor (M 2 , M 2   b ) comprising a control electrode, a current input electrode and a current output electrode is connected with the control electrode to the junction (n 1 ) between the collector of the input transistor (Q 1 ) and the second current source (M 1 ), with the current input electrode to the second supply voltage ( 3 ) and with the current output electrode to the anode of the first diode (Q 2 , Q 2   b ).

Description:
BACKGROUND OF THE INVENTION 
     The invention relates to an input buffer for a switched emitter follower-like track-and-hold amplifier comprising an input stage with an input transistor, a first diode, a cathode side of which first diode is connected to an emitter of the input transistor, a first current source between on the one hand the junction between the cathode of the first diode and the emitter of the input transistor and on the other hand a first supply voltage line, an anode of the first diode being connected to a track-and-hold controlled emitter follower. 
     Such an input buffer is known from U.S. Pat. No. 5,583,459. Input buffers described therein have a current source connected between the anode of the first diode and a second supply voltage. The presence of a diode is crucial to disconnect the input signal during the hold-mode. 
     It is known to embody the current source between the anode of the first diode and the second voltage supply with MOSFETs. A problem with such a current source embodied with MOSFETs is the nonlinear junction impedance thereof. When high input frequencies are used as input signals to this input buffer it is especially the nonlinear junction capacitance that plays a prominent role, at lower frequencies the nonlinear junction resistance also will play a role. The nonlinear junction impedance leads to distortion of the signal at the anode of the first diode. That distortion cannot be prevented by increasing the current through the current source since more current requires a proportionally larger current source with inherently larger nonlinear junction impedance. Furthermore a strict trade-off exists between available voltage headroom, noise and nonlinear output impedance of the current source. 
     SUMMARY OF THE INVENTION 
     It is an object of the present invention to provide an input buffer in which the trade-off between voltage headroom, noise and nonlinear output impedance has substantially decreased. 
     An input buffer according to the invention thereto is characterised in that the first current source is a non-switched constant current source, in that a second current source is connected between a collector of the input transistor and a second supply voltage, in that a second transistor comprising a control electrode, a current input electrode and a current output electrode is connected with the control electrode to the junction between the collector of the input transistor and the second current source, with the current input electrode to the second supply voltage and with the current output electrode to the anode of the diode. 
     Thereby it is achieved that a nonlinear output impedance of the second transistor does not substantially interact in the signal path. Any nonlinear current flows through the second transistor itself. A consequence of connecting the second transistor according to the invention is that the junction between the collector of the input transistor and the second supply voltage has an AC content that is attenuated relative to the input AC content by the gain of the second transistor. Consequently a good current source, also in terms of noise, can be selected as the second current source. 
     In a preferred embodiment of an input buffer according to the invention the second current source comprises a MOSFET. 
     Thereby it is achieved that a threshold voltage is available for V GT  of that MOSFET. 
     In a further preferred embodiment of the invention the second transistor is a MOSFET. 
     Thereby it is achieved that the voltage at the anode of the first diode can reach up to the V GT  of the second transistor. That is of advantage in situations where the second supply voltage is a low voltage. 
     The trade-off between voltage headroom, noise and nonlinear output impedance, known from the prior art, has substantially decreased in an input buffer according to the invention. 
    
    
     BRIEF DESCRIPTION OF THE DRAWING 
     The invention shall now be described with reference to the accompanying drawings in which: 
     FIG. 1 shows a first embodiment of an input buffer according to the present invention; 
     FIG. 2 shows a second embodiment of an input buffer according to the present invention. 
     FIG. 3 shows a third embodiment of an input buffer according to the present invention. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     FIG. 1 shows an input buffer  1  connected between a first supply voltage line  2  and a second supply voltage line  3 . An input transistor Q 1  has its emitter connected to a first side of a first current source  4 . The second side of current source  4  is connected to supply voltage line  2 . The emitter of transistor Q 1  is also connected to an emitter of transistor Q 2 . A base of transistor Q 2  is connected to a collector of transistor Q 2 . The collector of transistor Q 2  is connected to a drain of a MOSFET M 2 . A source of MOSFET M 2  is connected to supply voltage line  3 . A gate of MOSFET M 2  is connected to a collector of transistor Q 1 . The junction between the collector of transistor Q 1  and the gate of MOSFET M 2  is connected to a drain of a MOSFET M 1 . A source of MOSFET M 1  is connected to supply voltage line  3 . A gate of MOSFET M 1  is connected through a line  5  to a source of constant voltage (not shown). In the present embodiment transistor Q 2  has been described with its base connected to its collector. Functionally thereby transistor Q 2  operates as a diode, the cathode of which corresponds to the emitter of transistor Q 2  and the anode of which corresponds to the base and collector of transistor Q 2 . It is to be noted that in the present invention the functioning of the input buffer is independent of the choice for a single diode at the position of transistor Q 2  or for a transistor with its base and collector connected together. In the claims any transistor that is wired and functions as a diode, like transistor Q 2  in FIG. 1 will be called a diode. 
     The base of transistor Q 2  is connected to a base of transistor  6 . A collector of transistor  6  is connected to the second supply voltage line  3 , an emitter of transistor  6  is connected to a collector of a transistor  7  and to a first side of a capacitor C H . A second side of capacitor C H  is connected to the second supply voltage line  3 . A base of transistor  7  is connected by a line  8  to a source of track signals T. An emitter of transistor  7  is connected to a first side of a current source  9 . A second side of current source  9  is connected to first supply voltage line  2 . The first side of current source  9  is also connected to an emitter of a transistor  10 . A base of transistor  10  is connected through a line  11  to a source of hold signals H. A collector of transistor  10  is connected to a junction between the base of transistor Q 2  and the base of transistor  6 . The junction between the collector of transistor Q 1 , the gate of MOSFET M 2  and the drain of MOSFET M 1  will be called node n 1  hereinafter. The junction between the base of transistor Q 2 , the base of transistor  6  and the collector of transistor  10  will be called node n 2  hereinafter. 
     MOSFET M 1  operates as a constant current source. Transistors Q 1  and Q 2  together with constant current source  4  operate to apply a current into node n 2  that is equal to the current input at the base of transistor Q 1 . MOSFET M 2  with its gate connected to node n 1  completes a loop L. Any nonlinear output impedance of MOSFET M 2  does not interact in the signal path: any nonlinear current flows through the MOSFET M 2  itself which operates as a feedback transistor. As a consequence node n 1  has an AC content that is attenuated related to the AC content of the input signal at the base of transistor Q 1  by the gain of MOSFET M 2 . It is to be noted that MOSFET M 2  operates preferably, but not necessarily in its saturation range. 
     It is to be noted that interaction in the signal path of any nonlinear output impedance of MOSFET M 1  has substantially decreased. The loop formed by MOSFET M 2 , transistor Q 2 , transistor Q 1  and current source  4  effectively decreases any consequences of nonlinearity in the output impedance of MOSFET M 1  such that AC signals at the input of Q 1  have been attenuated at node n 1  by the gain of MOSFET M 2 . 
     An input buffer according to FIG. 1 can accommodate large signals at the base of transistor Q 1 . A threshold voltage is available for VGT of MOSFET M 1 . Also the output node n 2  can reach up to VGT of MOSFET M 2 . 
     Though MOSFET M 2  has been described hereinbefore as a MOSFET it is also possible to make use of PNP transistors. Also although M 1  has been described as a single MOSFET it is possible to apply other kinds of current sources. 
     It is to be noted that the input buffer shown in FIG. 1 can hardly operate as a track-and-hold amplifier since during a hold-mode a voltage at node n 2  can not drop since transistor M 2  essentially drains all currents. Nevertheless it may function as an input buffer. 
     FIG. 2 therefore shows a further embodiment of an input buffer according to the invention which may indeed function as a track-and-hold amplifier. Like elements have been indicated by the same reference numerals as in FIG.  1 . Instead of transistor Q 2  two transistors Q 2   a  and Q 2   b  are now present both with their emitters connected to the emitter of transistor Q 1  and both with their bases connected to their collectors. The collector of transistor Q 2   a  is connected to a drain of a MOSFET M 2   a . A source of MOSFET M 2   a  is connected to supply voltage line  3  and a gate of MOSFET M 2   a  is connected to node n 1 . The collector of transistor Q 2   b  is connected to a drain of a MOSFET M 2   b . A source of MOSFET M 2   b  is connected to supply voltage line  3  and a gate of MOSFET M 2   b  is connected to node n 1 . The base of transistor Q 2   b  is connected to node n 2 . 
     It is to be noted that node n 2 , transistor  6 , capacitor C H , and switch  52  together form a well known switched emitter follower-like track-and-hold circuit, that is known in the art and the operation of which is considered to be known to a person skilled in the art and will therefore not further be described. 
     An extra branch of transistors M 2   a  and Q 2   a  keeps the loop intact and allows for the branch of transistors M 2   b  and Q 2   b  to be removed from the loop thereby allowing node n 2  to drop. Thus the loop L remains intact and is not interrupted, like in the circuit according to FIG. 1, during a hold-mode. In the embodiment according to FIG. 2 node n 2  has to drop in voltage to turn off transistors Q 2   b  and Q 3 . 
     A further advantage of an input buffer according to the invention is a substantial decrease in the signal path of a non linear base current which itself is due to collector-base modulation of transistor  6 . The collector of transistor  6  is at a fixed voltage, namely the voltage VCC at the supply voltage line  3 . During tracking there is a signal at node n 2  which is the same signal as the signal present at the base of transistor Q 1 . That signal because of the fixed voltage at the collector of transistor  6  leads to a nonlinear base current into transistor  6 . However, due to the loop L formed by the transistors M 2  and M 2   a /M 2   b , respectively these nonlinear base currents are effectively eliminated. 
     FIG. 3 shows a further embodiment of an input buffer according to the invention. Like elements have been indicated by the same reference numerals as in FIGS. 1 and 2. Instead of transistor Q 2  two transistors Q 2   a  and Q 2   b  are now present both with their emitters connected to the emitter of transistor Q 1  and both with their bases connected to their collectors. The collector of transistor Q 2   a  is connected to a drain of a MOSFET M 2   a . A source of MOSFET M 2   a  is connected to supply voltage line  3  and a gate of MOSFET M 2   a  is connected to node n 1 . The collector of transistor Q 2   b  is connected to a drain of a MOSFET M 2   b . A source of MOSFET M 2   b  is connected to supply voltage line  3  and a gate of MOSFET M 2   b  is connected to node n 1 . The base of transistor Q 2   b  is connected to node n 2 . Node n 2  is also connected to a collector of a transistor  12 . A basis of transistor  12  is connected to line  11 . An emitter of transistor  12  is connected to a first side of a current source  13 . A second side of current source  13  is connected to supply voltage line  2 . A junction between the emitter of transistor  12  and the first side of current source  13  is connected to an emitter of a transistor  14 . A base of transistor  14  is connected to line  8 . A collector of transistor  14  is connected to the junction between the emitters of transistors Q 1 , Q 2   a  and Q 2   b . Furthermore it is shown that the current source formed by MOSFET M 1  generates a current of strenght 1*I. Current source  4  generates a current of strength 2*I and current source  13  generates a current of strength 1*I. 
     Transistors  7  and  10  and current source  9  together form a switch S 2  and transistors  12  and  14  and current source  13  together from a switch S 1 . Switches S 1  and S 2 , transistor  6  and capacitor C H  together form a switched emitter follower topology track-and hold amplifier that is known in the art and the operation of which will not further be described inhere. 
     An extra branch of transistors M 2   a  and Q 2   a  keeps the loop intact and allows for the branch of transistors M 2   b  and Q 2   b  to be removed from the loop thereby allowing node n 2  to drop. Switch SI prevents that node n 1  drops in voltage when switching takes place from track mode to hold mode. By the choices of a current 1*I through M 1  and 2*I through current source  4  there is always a current of strenght 1*I through M 2   a  and Q 2   a . Thereby the loop L remains intact and is not interrupted like in the circuit according to FIG. 1 when during a hold node n 2  drops in voltage to turn off transistors  6  and Q 2 . In the embodiment according to FIG. 2 node n 2  has to drop in voltage to turn off transistors Q 2   b  and Q 3 . The switch S 1  delivers in the hold mode the current that is supplied by transistors M 2   b  to node n 2 . This helps pulling down the voltage on node n 2 . 
     A further advantage of an input buffer according to the invention is a substantial decrease in the signal path of a nonlinear base current which itself is due to collector-base modulation of transistor  6 . The collector of transistor  6  is at a fixed voltage, namely the voltage VCC at the supply voltage line  3 . During tracking there is a signal at node n 2  which is the same signal as the signal present at the base of transistor Q 1 . That signal because of the fixed voltage at the collector of transistor  6  leads to a nonlinear base current into transistor  6 . However, due to the loop L formed by the transistors M 2  and M 2   a /m 2   b , respectively these nonlinear base currents are effectively eliminated.