INTEGRATED PUSH-PULL OUTPUT STAGE

Abstract: An integrated push-pull output stage for generating a pulse-like output signal in dependence upon a pulse-like input signal includes complementary output field-effect transistors which each consist of two groups of parallel-­ connected subtransistors (P1 to P4; N1 to N4) each of the same conductivity type. Connected into the lead to each gate electrode of each of the subtransistors (P1 to P4; N1 to N4) in each group is a resistance element (TP0 to TP3; TN0 to TN3). Associated with each subtransistor (P1 to P4; N1 to N4) in each group is a disconnecting field-effect transistor (PD1 to PD4; ND1 to ND4). Connected in parallel with each group of subtransistors (P1 to P4; N1 to N4) is a field-effect transistor (MN, MP) of respective opposite conductivity type which is connected as source follower as regards the input signal and output signal.

Description: The invention relates to an integrated push-pull output stage for generating a pulse-like output signal in dependence upon a pulse-like input signal comprising complementary output field-effect transistors which in each case consist of two groups of parallel-connected subtransistors each of the same conductivity type. In integrated semiconductor circuits which have been developed specifically for obtaining high switching speeds parasitic inductances of the supply lines to the active circuit elements have proved particularly disadvantageous because they prevent implementation of the theoretically possible high switching speeds in practice. In particular, in the output stages of integrated circuits in which relatively high currents flow, in the parasitic line inductances in highspeed switching operations, thus for example in high speed switching between a high voltage level and a low voltage level, high voltage peaks are generated which lead to a strong overshooting at the pulses furnished by the respective output stage. Said overshooting can lead to erroneous output signals because particularly with the low signal value, which lies in the range of ground potential, an undesired increase of said potential occurs which by the subsequent circuit stages, in the most unfavourable case, leads to the signal furnished by the output stage being interpreted as signal with high value due to the overshoot phenomena although it should actually be a signal with low value. This phenomenon is referred to in English usage as "ground bounce". The overshoot can of course be reduced in that the switching edges of the pulse-like signals are made slower but this however is contrary to the objective of achieving high switching speeds. In EP-A-0 292 641 and in the book published by the company Texas Instruments "Advanced CMOS Logic, Designers Handbook", which came out in 1987, in chapter 3.1.5, page 3-9, output stages of the type defined in the precharacterizing part of claim 1 are described in which steps are taken to reduce the unfavourable overshoot. In these output stages the output transistors are each formed from a plurality of individual transistors which are connected in parallel with each other. The gate zones of said individual transistors are connected in series and act like a resistive line. Since this line also has a certain capacitance the series circuit of the gate zones acts like a delay line so that the signal from the first transistor to the last transistor of the parallel-connected individual transistors requires a certain travelling time. However, without special measures this delay is not large enough to slow the edges of the switching pulses down to such an extent that undesirable overshoots no longer occur. In the output stage disclosed in EP-A-0 292 641 no such special measures are used whereas in the output stage disclosed in the before-mentioned book the particular measure for reducing the overshoots includes the use of a current grounding or dissipating transistor which is connected to the series circuit of the gate zones of the output transistors of one of the two groups of individual transistors in the push-pull output stage and in the switched-on state causes a current to flow through the series circuit of the gate zones to ground. The flowing of the current through the series circuit of the gate zones results in voltage drops occurring at the individual gate zones which act like individual resistances so that the gate voltages diminish consecutively from the first transistor of the group to the last. Due to the voltage drops occurring at the individual gate zones this additional current through the series circuit of the gate zones caused by the current grounding transistor opposes the formation of the overshoots. The current dissipating or grounding transistor must however be rendered nonconductive again after a defined time to prevent current permanently flowing through the gate zones. This blocking is carried out in dependence upon the input signal of the output stage from which by means of a delay circuit a control signal is generated which after the delay generated by the delay circuit renders the current grounding transistor nonconductive again. In the most favourable case, i.e. when the production-inherent parameters of the output stage have good values, the delay circuit must keep the current grounding transistor conductive long enough for the current to flow through the series circuit of the gate zones long enough for the desired effect of the gate voltage reduction to be maintained for an adequately long time. This current flow and the resulting voltage drop at the gate zones is the condition necessary for the undesired overshoots to be greatly reduced. If however the production-inherent parameters of the integrated output stage have unfavourable values the travelling time of the signal through the gate zones is from the start already relatively long and consequently the current additionally caused by the current grounding transistor together with the resulting further increase of the travelling time leads to excessive slowing down of the edges of the output signal of the output stage and this is extremely undesirable because the current grounding transistor, although it is intended to reduce the overshoot, should not excessively slow down the edge steepness of the output pulses. The invention is based on the problem of providing an integrated push-pull output stage in which the overshoot at the pulses of the output signal is substantially reduced without the switching speed of the output stage being appreciably impaired. According to the invention this problem is solved by the features defined in the characterising part of claim 1. Advantageous further developments of the invention are characterized in the subsidiary claims. The invention will now be explained by way of example with the aid of the drawing which shows a circuit diagram of the integrated push-pull output stage according to the invention. The output stage illustrated in the drawing is a CMOS push-pull output stage of which the output transistors are each divided into four subtransistors, i.e. into the P-channel transistors P1, P2, P3, P4 and the N-channel transistors N1, N2, N3, N4. As can be seen, the subtransistors of each group are connected in parallel to each other, a subtransistor of the one group being connected in each case in series with a subtransistor of the other group, i.e. for example the transistor P4 with the transistor N4. Between the gate electrodes of the parallel-connected subtransistors P1 to P4 and N1 to N4 resistance elements TP1, TP2, TP3 and TN1, TN2, TN3 are inserted. These resistance elements, as usual in integrated circuit technology, are each formed by two parallel-connected individual transistors, one of the transistors being an N-channel transistor and the other a P-channel transistor. The resistance element connected in front of the gate electrode of the respective first subtransistor P1, N1 consists in each case of a field-effect transistor TP0 and TN0 respectively although this resistance element could also be made up like the resistance elements TP1 to TP3 or TN1 to TN3. Lying in parallel with each group of subtransistors P1 to P4, N1 to N4 is a transistor MN and MP respectively which is connected as source follower, which means that at its source electrode apart from the gate-source voltage occurring there as voltage drop the voltage applied to its gate electrode is emitted. Connected to each gate electrode of each subtransistor P1 to P4 and N1 to N4 is a disconnecting or switch-off transistor PD1 to PD4 or ND1 to ND4 respectively which in certain operating phases ensured reliable disconnection of the respective subtransistors. The output stage described behaves as follows: It will be assumed that at the input terminal 10 of the output stage a signal of low voltage value is applied. In this state the transistor ND is nonconductive whilst the transistor PD is in the conductive state. The transistors PD1 to PD4 are conductive whilst the transistors ND1 to ND4 are nonconductive. At the output terminal 12 of the output stage there is also a signal with low voltage value. The transistors MN and MP are likewise blocked because between their gate and source electrodes there is no potential difference as would however be necessary to set these transistors to the conductive state. The transistors P1 to P4 are blocked due to the conductive state of the transistors PD1 to PD4 whilst the transistors N1 to N4 are held in the conductive state. It is now assumed that the input voltage at the input terminal 10 rises so that the transistor ND becomes conductive. At the same time the transistors PD1 to PD4 are rendered nonconductive. It is presumed here that the voltage rise at the input terminal 10 is with steep edge. The resistance elements TP0 to TP3 formed by transistors form together with the gate capacitances of the transistors P1 to P4 a divided RC network which acts like a delay line. This means that with the rise of the voltage value of the signal at the input terminal 10 and the passage of the transistor ND to the conductive state the transistors P1 to P4 do not become conductive simultaneously but consecutively with a certain delay and as a result the voltage jump of the input signal is reproduced in the output signal only in retarded form. Since the transistors P1 to P4 become conductive consecutively it is not necessary for a large current to start flowing in very short time; instead, smaller subcurrents start to flow consecutively and as a result in the lines connected to the output terminal the current changes per unit time are smaller so that the voltages induced in the line inductances are reduced. Since the resistance elements TP0 to TP3 are made up of transistors they are subjected largely to the same parameter fluctuations of the manufacturing process as the transistors P1 to P4 acting as capacitances. If the resistance elements TP0 to TP3 become of higher resistance because of a process deviation then the gate capacitances of the transistors P1 to P4 arising under the influence of the same process deviations necessarily become smaller so that the product of resistance and capacitance, that is the RC time constant of the aforementioned RC network, remains approximately constant. The transistor MN connected in parallel with the subtransistors P1 to P4 substantially contributes to avoiding excessive overshoot when switching the input signal from a low voltage value to a high voltage value. The transistor MN receives the input signal directly at its gate electrode so that it passes to the conductive state after the input signal has exceeded its threshold voltage value. Since the voltage at the output terminal 12 reaches the high voltage value only with delay a voltage difference between the gate electrode of the transistor MN and its source electrode connected to the output terminal 12 is present only during the leading edge of the input signal so that said transistor MN can also only conduct current for as long as the input signal varies. If however a constant voltage value is present at the input terminal 10 the transistor MN cannot conduct any current because of the identitiy of the voltages at its gate electrode and at its source electrode. The use of the transistor MN connected as source follower has a further advantage. When the circuit has lower resistance parameters due to the process or when at the output 12 a smaller (more resistive) load is present due to the resultant steeper edge of the output signal the transistor MN is effective for a shorter period than with more resistive circuit parameters and greater (less resistive) load. This feedback effect occurs because the transistor MN, as mentioned above, is effective in dependence upon the voltage difference between its gate electrode and its drain electrode. Since in accordance with the above explanation the transistor MN can only be effective during the transition of the input signal from the low voltage value to the high voltage value, this transistor influences the profile of the leading edge of the signal at the output terminal 12. The transistor MN connected as source follower makes the edge steeper in the centre region of the rise without however any danger of overshoot occurring because it goes into the nonconductive state again as soon as the voltage at the output terminal 12 differs from the voltage at the input terminal 10 by less than the gate-source voltage drop occurring at the transistor MN. In the case of the transition of the signal at the input terminal 10 from the low voltage value to the high voltage value the transistors ND1 to ND4 act as disconnecting transistors for the transistors N1 to N4 because with the rise of the input voltage they apply to the gate electrodes of the transistors N1 to N4 the ground signal which reliably blocks said transistors so that only the transistors P1 to P4 can be effective. The connecting together of the transistors P1 to P4 with the resistance elements TP0 to TP3 to form a distributed RC network with the effect of a delay line makes it possible to work with a steep-edged transition from the low voltage value to the high voltage value at the input terminal 10 because said RC network reduces the edge steepness at the edge start and edge end. The transistor MN ensures that this does not lead to an appreciable retardation of the edge as a whole because as explained above this transistor makes the edge steeper in its centre region. The description so far related to the transition of the voltage at the input terminal 10 from a low value to a high value. It applies accordingly to the transition from a high value to a low value, the respective complementary transistors with their corresponding behaviour then being active. In this case the transistors N1 to N4 with their associated resistance elements TN0 to TN3 act as distributed RC network with the effect of a delay line and the transistor MP connected in parallel with the transistors N1 to N4 takes over the action described above of the transistor MN. The circuit described thus permits the generation of a steep-edge output signal and the edge steepness is achieved both with ascending and descending edges. In spite of the edge steepness an undesirable overshoot is avoided.

Claims: Integrated push-pull output stage for generating a pulse-like output signal in dependance upon a pulse-like input signal comprising complementary output field-effect transistors which in each case consist of two groups of parallel-connected subtransistors (P1, P2, P3, P4; N1, N2, N3, N4) each of the same conductivity type, wherein into the lead to each gate electrode of each of the subtransistors (P1, P2, P3, P4; N1, N2, N3, N4) in each group a resistance element (TP0, TP1, TP2, TP3; TN0, TN1, TN2, TN3) is inserted, where in the drains of the subtransistors (P1, P2, P3, P4; N1, N2, N3, N4) of the two groups of transistors are connected with the output terminal (12), characterized in that with each subtransistor (P1, P2, P3, P4; N1, N2, N3, N4) in each group a disconnecting field-effect transistor (PD1, PD2, PD3, PD4; ND1, ND2, ND3, ND4) is associated, which is connected between the gate of the associated subtransistor (P1, P2, P3, P4; N1, N2, N3, N4) and the respective power supply lines and is controlled by the input signal, and that with each group of subtransistors (P1, P2, P3, P4; N1, N2, N3, N4) a further field-effect transistor (MN, MP) of respective opposite conductivity type is connected in parallel and with respect to the input signal and the output signal is connected as source follower, the gates of said furhter field effect transistor (MN, MP) being connected with the input terminal (10) such that said further field effect transistor is only conducting during an input signal transition. Integrated push-pull output stage according to claim 1, characterized in that the resistance elements (TP1, TP2, TP3; TN1, TN2, TN3) which are inserted between the gate electrodes of consecutive subtransistors (P1, P2, P3, P4; N1, N2, N3, N4) are formed by two complementary parallel-connected field-effect transistors. Integrated push-pull output stage according to claim 1, characterized in that the disconnecting transistors (PD1, PD2, PD3, PD4; ND1, ND2, ND3, ND4) are respectively of the same type as the subtransistors (P1, P2, P3, P4; N1, N2, N3, N4) with which they are associated and are connected to the gate electrode of the respective subtransistor (P1, P2, P3, P4; N1, N2, N3, N4).

Inventors: JUNGERT HORST, MUELLER MANFRED, JUNGERT, HORST, MUELLER, MANFRED, Müller, Manfred 
Applicants: TEXAS INSTRUMENTS DEUTSCHLAND, TEXAS INSTRUMENTS DEUTSCHLAND GMBH
Application year: 1990
Main classification: H03K5
IPC codes: H03K19, H03K17, H03K5