Technical Field

This invention relates to CMOS to ECL interface circuits, of the kind including input means adapted to receive CMOS logic level signals and output means adapted to provide ECL logic level signals.

Background Art

U.S. Patent No. 4,453,095 discloses a CMOS to ECL interface circuit for a five volt CMOS voltage swing, including a conventional CMOS inverter having an output coupled to the base electrode of an NPN transistor, which provides an ECL logic swing output at the emitter, which is coupled via termination resistor to a voltage supply source. The U.S. Patent also discloses a CMOS logic circuit for a two volt logic swing, including a conventional CMOS inverter having an output connected to a one input of a differential pair of bipolar transistor. The differential pair has an output connected to the base of a bipolar transistor whose emitter provides the desired ECL output.

Disclosure of the Invention

It is an object of the present invention to provide a CMOS to ECL interface circuit of the kind specified which is of simple construction, thereby minimizing propagation delays.

Therefore, according to the present invention, there is provided a CMOS to ECL interface circuit of the kind specified, characterized by first and second field effect transistors having respective first and second source-drain paths connected in series between a supply voltage source and a reference potential source, in that said second field effect transistor is connected to operate in a body effect

mode, and in that the junction point between said first and second source-drain paths is coupled to said output means, the arrangement being such that, in response to a first CMOS logic level input signal applied to said input means, said first field effect transistor is operative to cause said output means to provide a first ECL logic level output signal, and in response to a second CMOS logic level input signal applied to said input means, said second field effect transistor is rendered operative in said body effect mode to cause said output means to provide a second ECL logic level output signal.

It will be appreciated that a CMOS to ECL interface circuit according to the invention has the advantage that conversion of CMOS input levels which have a voltage swing of 3 volts is achieved with minimum propagation delay. Further advantages are that the interface circuit requires a minimum of integrated circuit area and uses relatively little power.

It should be understood that the present invention makes use of the body effect operative in the second transistor to achieve the desired output ECL voltage levels. As is known, the body effect, which is sometimes referred to as the back-gate bias, is effective for a transistor whose source electrode is ungrounded, to increase the threshold voltage of the transistor, that is, the gate voltage necessary to cause channel inversion.

Brief Description of the Drawings

Three embodiments of the present invention will now be described by way of example with reference to the accompanying drawings, in which:-

Fig. 1 is a circuit diagram of one embodiment of the invention;

Fig. 2 is a circuit diagram of a second embodiment of the present invention; and

Fig. 3 is a circuit diagram of a third embodiment of the present invention.

Best Mode for Carrying Out the Invention

Referring to Fig. 1, four field effect transistors 21, 22, 23 and 24 are connected in series by their source and drain electrodes between a refer¬ ence potential (ground) and a -3 volt DC power source. In the preferred embodiment, transistors 21, 22 and 23 are p-channel types and transistor 24 is of the n- channel type. The gate electrodes of transistors 21 and 24 are connected to an input terminal 10. The input terminal 10 is connectable to receive the logic level signal, Vi _n , from a CMOS device, which signal has logic levels that range between -3 volts and 0 volts. The gate electrode of transistor 22 is con¬ nected to its drain electrode and to an output termi¬ nal 60. The gate electrode of transistor 23 is con¬ nected to its drain electrode. The output terminal 60 is connected to a -2 volt DC power source by a termination resistor 62. The output voltage level, ^v out' available on the output terminal 60 will range from -0.88 volts to -1.8 volts.

In the present circuit, the transistors 22 and 23 are operated in their saturation region which causes the transistors to act as diodes. The opera¬ tion of the remainder of the circuit, for DC opera¬ tion, is as follows: When the signal V_ _n is -3 volts, the n-channel transistor 24 is turned off and the p- channel transistor 21 is turned on. The voltage on the output terminal 60 charges up to a voltage level of -0.88 volts which is a "HI" level input for ECL logic. When the input voltage Vi _n equals 0 volts, the transistor 21 is turned off and transistor 24 is turned on thereby operating as a complementary switch.

The voltage on the output terminal 60 then experiences a discharge to -1.8 volts which is a "LO" level input for ECL logic. This effect is caused somewhat by the body effect of transistor 23, which, as is known, increases the transistor threshold voltage when the source electrode is ungrounded. Thus, due to the body effect, the threshold voltage of the transistor 23 is greater than the threshold voltage of the transistor 22. The value of the termination resistor 62 may be 50 to 100 ohms.

Referring now to Fig. 2, wherein a second embodiment of the invention is disclosed, the input terminal 10 receives the CMOS logic level input signal and connects the signal to the gate electrode of the field effect transistors 31, 32, 33 and 36. The source electrode of field effect transistor 31 is connected to a reference potential, such as ground. The drain electrode of transistor 31 is connected to the source electrode of transistor 34 and to the gate electrode of transistor 35. The drain electrode of transistor 34 is connected to. an output terminal 60 and to the drain and source electrodes of transistors 35 and 36, respectively. The source electrode of transistor 35 is connected to the reference potential. The drain electrode of transistor 36 is connected to a -3 volt DC power source. The source electrode of transistor 32 is also connected to the reference potential. The drain electrode of transistor 32 is connected to the drain electrode of transistor 33 and to the gate electrode of transistor 34. The source electrode of transistor 33 is connected to the -3 volt DC power source. Thus, the transistor pair 32, 33 is connected to operate as an inverter. The output terminal 60 is connected to a -2 volt DC power supply by means of a termination resistor 62.

In operation, when the voltage on the input terminal 10 equals 0 volts, transistors 31, 32 and 36

are turned off. Transistor 33 is turned on, thereby turning on transistors 34 and 35 which in turn pulls the output terminal voltage, V _ou t, upwards to -0.88 volts. When the voltage Vj_ _n , on the input terminal, goes to -3 volts, transistor 34 is turned off and transistor 31 is turned on turning off transistor 35, and transistor 36 is turned on, which in turn pulls down the voltage, V _ou t, on the source electrode (output terminal 60) of transistor 36 to -1.8 volts in view of the body effect operative in the transistor 36. The transistors 35 and 36 operate in complementary fashion to alternately connect the output terminal to either the reference potential (ground) or to the CMOS compatible power supply (-3 volt DC) in response to the CMOS logic level signals coupled from the input terminal by field effect tran¬ sistors 31-34.

The Fig. 2 circuit embodiment is noninverting, that is, as the input signal goes more positive, the output signal also goes more positive, and conversely, when the input signal goes more nega¬ tive, the output signal will also go more negative. It is sometimes more preferable to have an inverting type circuit for interfacing two logic levels and such circuits are disclosed in the Fig. 1 and Fig. 3 embodiments.

Referring now to Fig. 3, the input terminal 10 is coupled to the gate electrode of transistors 42, 43 and 44. The transistor 42 has its source electrode connected to a reference potential, such as ground, and its drain electrode connected to the gate elec¬ trode of transistor 41, the gate electrode of transis¬ tor 46, and the drain electrode of transistor 43. The source electrode of transistor 43 is connected to a -3 volt DC power source. Thus, the transistor pair 42, 43 is connected to operate as an inverter. The source electrodes of transistors 41 and 45 are connected to a

reference potential and the drain electrode of transistor 41 is connected to the source electrode of transistor 44 and the gate electrode of transistor 45. The drain electrode of transistor 44 is connected to the drain electrode of transistor 45 and to the source electrode of transistor 46. The drain and source electrodes of transistors 45 and 46 respectively are connected to the output terminal 60 which in turn is connected to a -2 volt DC power source, by means of a termination resistor 62. The drain electrode of transistor 46 is connected to the -3 volt DC power source.

In operation, when the voltage, V _n , on the input terminal 10 goes to 0 volts, transistor 42 is turned off and transistor 43 is turned on. When transistor 43 is on, it turns on transistor 46 and turns on transistor 41. Transistor 44, being off, in turn causes transistor 45 to go off. This in turn, lowers the voltage, _ou -t, on the output terminal 60 to -1.8 volts in view of the body effect operative in the transistor 46. When the voltage on the input terminal 10 goes to -3 volts, transistor 42 is turned on and transistor 43 is turned off. In addition, transistor 44 is turned on. With transistor 43 off, transistor 41 is turned off and transistor 45 is turned on. Transistor 46 tracks transistor 41 and is therefore turned off. With transistor 46 off and transistor 45 on, the voltage, _ou t, on the output terminal 60 rises to approximately -0.88 volts. It can thus be seen that for a 0 volt input there is a - 1.8 volt output and for a -3 volt input there is a relatively positive output of

-0.88 volts. Therefore, the Fig. 3 circuit embodiment is an inverting circuit.

The circuit embodiments of the present invention provide a novel solution for interfacing CMOS logic level signals to ECL logic level signals

while minimizing signal propagation delays and the use of semiconductor area.