Source follower CMOS input buffer

Abstract: A source follower CMOS input buffer includes an inverter input stage (34) comprising a p-channel transitor (36) and an n-channel transistor (38) forming a CMOS pair. The output of the inverter stage (34) is coupled to a field effect transistor (42) arranged as a source follower, and the source of the source follower transistor (42) is coupled to an inverter output stage (50,52) which also comprises a CMOS pair of transistors. The source follower transistor (42) enables the input buffer to drive high capacitances whilst having a low standby current.

Description: The present invention relates to an input circuit for use preferably in a CMOS semiconductor memory. An input buffer for a semiconductor memory should be able to receive a TTL input signal (being in the range of 0.8 to 2.0 volts) and generate an output signal in a voltage range corresponding to the chip (generally between VSS and VCC), which in the case of CMOS memories today is 0 volts to 5 volts. Another requirement is that the input buffer must be able to drive high internal chip capacitances. The problem which is solved by the present invention will be understood from consideration of the conventional input buffer circuit illustrated in Figure 1 and its shortcomings. The problem with the circuit of Figure 1 concerns the compromise made between speed of operation and standby current. As will be explained, these have been mutually inconsistent goals, i.e. to achieve increased speed, one sacrifices standby current. The present invention overcomes this problem. Figure 1 shows a conventional CMOS input buffer circuit 10 formed of field effect transistors (FETs) which are all enhancement mode devices. Circuit 10 receives an input at 12 and applies it to a first inverter stage 14 comprising a CMOS pair of transistors 16, 18. Device 16 is a p-channel device, whereas device 18 is an n-channel device. The source-drain paths of transistors 16, 18 are connected in series between VCC and ground, as illustrated. Input 12 is coupled to the gate electrodes of each of these transistors. A node A between the source of transistor 16 and the drain of transistor 18 provides an input to the second stage inverter 20. Inverter 20 comprises another pair of CMOS transistors including transistors 24 and 26 connected in a fashion identical to stage 14. The output node B of stage 22 is provided to the capacitive load of the chip as represented by capacitor 28. In Figure 1, if the input voltage is 2.0 volts, then transistors 16 and 18 will both be turned on. Current will flow from VCC to ground through the source-drain paths of these transistors. Transistor 18 will be sized approximately four or five times larger than transistor 16 in order to insure that the voltage at node A is at or near ground when the input voltage is at a TTL logic "1" level of about 2.0 volts. In actuality, the voltage at node A may be about 1.0 volts at this condition, and as a result, some current will flow in the second stage 20. The output of inverter stage 20 will have the same logic value as input 12. Consider that the capacity of the load is about 5 picofarads (pf). To drive this capacity, the transistors in output stage 20 will need a channel width typically of about 200 microns, using a guideline of about 40 microns per pf for an n-channel transistor. This is a fairly large transistor, and the transistors in the first stage 14 must be large because node A must drive each of transistors 24 and 26. The current through the first stage will be significant. To reduce this standby current, more inverter stages could be added. This would allow downsizing the transistors in the first stage. However, adding stages will increase the delay in the transitions in the input and the output. This undesirable result is overcome by the present invention. Reference is made to an article in IBM Technical Disclosure Bulletin Vol. 26 No. 8 January 1984, pages 4139-4140 by J.P. Bansal entitled "Inverting/non-inverting on-chip CMOS line driver". This discloses an inverting CMOS line driver and has an N-channel source follower between two CMOS inverter stages. The output stage is driven directly by the input stage. This reference is not concerned with providing a CMOS input buffer for receiving TTL logic level signals and converting them to CMOS logic level signals. Furthermore, an article entitled "Schnelle CMOS - Logikbausteine" by K. Schrack published in ELEKTRONIK, Vol. 32, No. 12, 16th June 1983, pages 63-66 describes conversion from LS-TTL logic to HC-CMOS logic using a TTL/CMOS level changer with open drain or open collector output. In one case a pull up resistor is used to alter the voltage level; in another, a special protection circuit is incorporated. None of the proposals in this paper provides a simple converting buffer circuit which can operate at high speed and with low power consumption. The present invention provides a buffer circuit for converting TTL logic levels to CMOS logic levels comprising an input stage configured as a CMOS inverter connected between a source of operating voltage and a reference voltage and including an input for receiving a TTL signal and an output; and an output stage configured as a CMOS inverter connected between the source of operating voltage and the reference voltage and including an input and an output wherein said buffer circuit is operable to receive TTL signals at said input of the input stage and convert them to CMOS levels provided at said output of said output stage, characterised by: a first n-channel field effect transistor connected as a source follower with its gate connected to the output of said input stage and connected between a source of operating voltage and the input of said output stage to drive said output stage; a second n-channel transistor connecting the source of said source follower transistor to a second reference voltage and having its gate electrode connected to receive said TTL signal; a p-channel field effect transistor connected between the source of operating voltage and said input of said output stage, and being gated by a CMOS signal having the same logic value as said TTL signal, and being connected to be turned on after said n-channel source follower transistor turns on; and a further inverter coupled to said input stage and providing an output connected to the gate electrode of said p-channel transistor. A preferred embodiment uses a p-channel transistor of small transistor width compared with the n-channel source follower transistor to assure that a full VCC level is achieved at the output of the pull up n-channel device. The advantage of this circuit is that regardless of whether a TTL or CMOS voltage level is applied at the input, the circuit according to this invention will have low standby current. Additionally, if one allows the circuit according to the present invention to draw the same current as the circuit of Figure 1, there will be an improvement in speed of the circuit. On the other hand, if one configures the circuit elements to achieve the same speed, a circuit according to the present invention will use about one-half the current of the circuit of Figure 1. For a better understanding of the present invention, and to show how the same may be carried into effect, reference will now be made by way of example to the accompanying drawings in which:- Figure 1 illustrates a circuit according to the prior art; and Figure 2 illustrates a circuit according to the present invention. In Figure 2, a circuit 30 receives an input at 32 and applies it to an input stage 34 which comprises transistors 36 and 38 which are enhancement mode devices, as are all of the other transistors illustrated in Figure 2. Transistor 36 is a p-channel device and transistor 38 preferably is its corresponding n-channel device. Transistors 36, 38 form a CMOS pair. As with stage 14 of Figure 1, the source-drain paths of transistors 36 and 38 are coupled in series between VCC and ground. An output 40 of inverter stage 34 is applied to the gate electrodes of an n-channel transistor 42 and a CMOS pair of transistors 44,46. Preferably, the channel width of transistors 44, 46 is small, illustratively 6 microns, whereas the width of n-channel transistor 42 illustratively will be about 120 microns. As a result, most of the gain of input stage 34 appearing on output node 40 will be directed to the gate electrode of transistor 42. The source-drain path of transistor 42 is coupled at the drain to VCC and at the source to a node C. A further transistor 48 is also an n-channel device whose source-drain path couples node C to ground. The gate electrode of transistor 48 is not, however, coupled to node 40, but instead is driven by the input 32. Thus transistor 42, coupled as a source follower to the first inverter stage, provides substantial pull-up to enable node C to drive more capacitance than node 40 would be able to drive. Additionally, the delay between a rising transition at node 40 and the rising transition at node C will be small because transistor 42 is connected as a source follower. Thus, node C closely follows node 40 because source follower transistor 42 is large and has a high gain. Node C drives an output inverter stage and is coupled to the gate electrode of a CMOS pair of transistors 50 and 52 whose source-drain paths are coupled in series between VCC and ground. A node D between the source of transistor 50 and the drain of transistor 52 provides the output to the capacitive load 54. It is well known that an n-channel transistor has more gain than a p-channel transistor of the same size. With the large pull-up of transistor 42, transistors 50 and 52 in the output stage can be made larger than otherwise obtainable by the prior art circuit of Figure 1. The larger width of transistors at this output stage will result in this stage having a faster operation than otherwise obtainable. As mentioned, the source follower transistor 42 is an n-channel device. As such, it cannot pull up all the way to VCC, but instead rises to one voltage transistion lower than VCC. To allow the voltage at node C to rise all the way to VCC, it is preferred to include a p-channel transistor 56 having its drain coupled to VCC and its source coupled to node C. This device is not added for speed, but only to ensure that the voltage rises fully at node C. Thus, it can be a small transistor, having a channel width illustratively of 6 microns. It does not affect the delay between the input 32 and output node D. Transistors 44 and 46 provide an inverter stage to allow the voltage at its output node E, which is coupled to the gate electrode of transistor 56, to be the logic inverse of the state of node 40. Transistors 44, 46 can be small, illustratively with a channel width of six microns, and the current drawn by them is correspondingly small. Transistor 48 is included in the preferred embodiment to insure that when the input voltage at 32 is 2.0 volts, node C will be pulled all the way to ground. Thus, transistor 48 is an n-channel device which is gated by the input voltage on 32. The source-drain path of transistor 48 will insure that node C drops to ground during this conditon. The benefit of this is that if the input voltage at 32 is at a TTL logic high of 2.0 volts, then the 0 volts at node C will insure that CMOS pair 50, 52 of the output inverter stage will draw no current. Thus, the operation of the circuit of Figure 2 ensures that the voltage at node C will either be ground (VSS) or VCC when the input voltage is high or low. Because of this, either transistor 50 or 52 will be off so there is no standby current consumed at the output stage. It may be noted at this point that in comparing the present invention to the conventional circuit illustrated at Figure 1, if the first inverter stage were to drive a second inverter stage of 120 microns total width (which is the width of transistor 42 in Fig. 2), then the following would be dictated. Because a p-channel device of a CMOS pair must be double the width of an n-channel device, the p-channel device would have a width of about 80 microns and the n-channel device would have a channel width of about 40 microns. The gain of this CMOS pair, arranged as an inverter, would be less than the gain of transistor 42 because the individual widths are smaller. Further, a p-channel device has about one-half of the gain of an n-channel device of the same width. Thus, the gain on pull-up of a CMOS pair connected as an inverter would be about one-third the gain of transistor 42, keeping the total channel width constant. Thus, the result of using an n-channel transistor rather than a CMOS inverter pair is to improve the pull-up by a factor of three. Illustratively, transistor 36 can have a width of 24 and a length of 4; transistor 38 can have a width of 120 and a length of 3.5; transistor 48 can have a width of 80; transistor 50 can have a width of 120; and transistor 22 can have a width of 240 (all expressed in microns). Using SPICE programming, the following Table I was generated which shows the standby current and delay times under various conditions. It will be seen that at a constant current of 0.3 milliamps, the circuit of Figure 2 provides a 1 ns improvement on down-going transitions and 0.6 ns improvement on upward transitions when the current is kept at a constant 0.3 mA. On the other hand, by comparing the current drawn by the Figure 1 circuit with that drawn by the Figure 2 circuit at substantially constant input-output transition delays, it can be seen that the circuit of Figure 2 draws 0.3 mA, whereas the circuit of Figure 1 draws 0.6 mA. Thus, the performance of the preferred embodiment of the present invention is consistently better than that of Figure 1 in terms of standby current and speed.

Claims: A buffer circuit for converting TTL logic levels to CMOS logic levels comprising an input stage (34) configured as a CMOS inverter connected between a source of operating voltage (VCC) and a reference voltage and including an input (32) for receiving a TTL signal and an output; and an output stage (50,52) configured as a CMOS inverter connected between the source of operating voltage (VCC) and the reference voltage and including an input and an output wherein said buffer circuit is operable to receive TTL signals at said input of the input stage (34) and convert them to CMOS levels provided at said output of said output stage, characterised by: a first n-channel field effect transistor (42) connected as a source follower with its gate connected to the output of said input stage and connected between the source of operating voltage (VCC) and the input of said output stage to drive said output stage; a second n-channel transistor (48) connecting the source of said source follower transistor (42) to the reference voltage and having its gate electrode connected to receive said TTL signal; a p-channel field effect transistor (56) connected between the source of operating voltage and said input of said output stage, and being gated by a CMOS signal having the same logic value as said TTL signal, and being connected to be turned on after said n-channel source follower transistor (42) turns on; and a further inverter (44,46) coupled to said input stage and providing an output connected to the gate electrode of said p-channel transistor (56). A buffer circuit as claimed in claim 1, in which the first n-channel field effect transistor (42) has the largest channel width of all devices (42,44,46) driven by the input stage (34). A circuit as claimed in claim 1 or 2, wherein said further inverter comprises a pair of CMOS transistors (44,46). A circuit as claimed in any preceding claim, wherein said second n-channel transistor (48) has a source-drain path which connects ground to the source of said source follower transistor (42). A circuit as claimed in any preceding claim, wherein said CMOS inverter input stage (34) comprises an n-channel transistor (38) and a p-channel transistor (36), the channel width of the n-channel transistor being of the order of 4 or 5 times larger than that of the p-channel transistor. A circuit as claimed in any preceding claim, wherein said output stage inverter comprises a CMOS pair of field effect transistors (50,52) each having a gate electrode which is connected to the source of said source follower transistor (42). A circuit as claimed in any preceding claim, wherein all the transistors are enhancement mode devices.

Inventors: ALLAN JAMES D, ALLAN, JAMES D.
Applicants: THORN EMI NORTH AMERICA, THORN EMI NORTH AMERICA INC.
Application year: 1986
Main classification: H03K19
IPC codes: H03K19, H01L21, H01L27