VOLTAGE SOURCES

Abstract: A circuit for generating voltages having values proportional to the threshold voltages (VT) of n-channel transistors used in the circuit comprises a current mirror M2, M3 having a reference current input generated from a reference voltage of value 2VT by an n-channel transistor M1. The output reference voltage of value 2VT by an n-channel transistor M4 whose gate is coupled either to its drain, for output voltages greater than VT, or to the gate of transistor M1 for output voltages less then VT.

Description: Voltage Sources This invention relates to voltage sources and particularly to circuits which provide specific voltages which are dependent on the threshold voltage of transistors used in the circuit. Such circuits are particularly useful in the field of CMOS IC's where it is advantageous to provide specific voltages whose values are proportional to the threshold voltage V τ of the transistors used therein. Such transistors may be either n- or p-channel field-effect transistors. One application is in logic circuits where threshold voltage dependent voltages are required in order to switch the transistors in the circuit so that logical decisions are made by the circuit. Another application is in sensing amplifiers in which lines connected to the inputs of the amplifier are precharged by voltages proportional to the threshold voltage in order to improve the sensitivity of the amplifier. Therefore it is an object of the invention to provide a circuit which generates voltages whose values are proportional to the threshold voltage of the transistors used in the circuit. Accordingly, the invention provides a voltage source circuit comprising a current mirror having an input and an output and coupled to a first reference potential line; a reference current source coupled to the current mirror input; and a bias transistor having a first current electrode coupled to the current mirror output, a second current electrode coupled to a second reference potential line and a control electrode coupled so as to produce at its first current electrode a voltage dependent on the reference current, wherein said current mirror output forms an output of the voltage source circuit. Preferably the reference current source comprises a transistor having a first current electrode coupled to said current mirror input, a second current electrode coupled to said second reference potential line and a control electrode for receiving on input reference voltage. As will be more fully described below, the control electrode of the bias transistor may be coupled to received either the input reference voltage or the voltage level at the current mirror output, depending on the required output from the voltage source circuit. The invention will now be more fully described by way of example with reference to the drawings of which: Figure 1 shows a circuit diagram of a basic embodiment of a voltage source circuit according to the invention; and Figure 2 shows a circuit diagram of an improved embodiment of a voltage source circuit according to the invention. Thus, Figure 1 shows a circuit diagram of a voltage source circuit providing voltages which are dependent on the threshold voltage of n-channel transistors. It comprises a current mirror composed of p-channel transistors M 2 and M 3 each having one current electrode coupled to a voltage supply line V DD . Transistor M 2 is diode-coupled with its second current electrode coupled to its gate electrode which is also coupled to the gate electrode of transistor M 3 . The input to the current mirror comprises the second current electrode of transistor M 2 which is coupled to the first current electrode of an n-channel transistor M 1 . This transistor has its second current electrode coupled to a ground reference potential line and its gate electrode coupled to receive an input reference voltage V-^p. In this embodiment of the voltage source circuit, the input re erence voltage is arranged to be twice the threshold N j , of the n-channel transistors. Thus: (0) V REF =2 V T Since the current I through a transistor having a threshold voltage Vm and biased by a voltage V is described by I=K (V-V τ ) 2 where is the transistor gain constant, the current through transistor Ml is This is the current input to the current mirror and the current output from_the mirror through transistor M 3 ls: I 3 = X I λ = X - j Vr 2 (2) where x is a constant determined by the geometry ratios of transistors M 2 and M 3 . The output of the current mirror is coupled to the drain of an n-channel bias transistor M 4 , this drain forming the output of the voltage source circuit. The source of transistor M 4 is coupled to the ground reference potential line and the gate of transistor M 4 is connected either to its own drain or the gate electrode of transistor M ] _ depending on the output voltage required from the voltage source circuit. If the gate electrode of transistor M 4 is coupled to its drain, its drain source voltage V 4 is determined by: ι 3 =κ 4 (v 4 -v τ ) (3) Rearranging this, gives: Substituting for I 3 from equation (2) gives : Thus the output voltage V 4 can be made to be any predetermined ratio of Vr j , greater than one by appropriately choosing K^/ Similarly, if the gate electrode of transistor M 4 is coupled to the gate electrode of transistor -^ , the transistor 4 can be made to operate in the triode region. In this case, the output voltage V 4 is given by: I 3 =K 4 [2 (2V T -V T ) V 4 -V 4 2 ] =K 4 (2V T V 4 -V 4 ^ ) ( 6 ) Substituting for I 3 from equation (2) gives: V 4 2 -2V T V 4 +XK 1 V T 2 / =0 (7) K 4 whose solution is: From this it can be seen that the output voltage V 4 can now be made to be lower than the threshold voltage V τ by appropriate choices of x, K^_ and K 4 . Thus, by coupling the gate of transistor M 4 to the gate of the transistor M lλ the ratio V 4 is less V T than one and by coupling the gate of transistor M to the is greater than one. Although the above calculations were performed for V REF ~ V T' ^ w^ 11 be appreciated that a similar result will be obtained for -^-. being any value (n+1) .V τ . In this case: so that for the gate of the transistor M 4 being coupled to its drain we have, similarly to equations (2) and (3) : I 3 = I-L = x K-LCnVrp) 2 = κ 4 (V 4 - V τ ) 2 Thus: (v 4 -v τ ) giving: " K j ' nV T =V 4 -V T y To generate a current in transistor M^, n must be greater than zero. However when V^p is generated by diode-connected transistors connected in series, to realise larger than two i.e. three or four or more, requ res higher values of the supply voltage DD . Therefore a useful compromise is to set 2V T . One circuit in which a voltage V^-, with a value of approximately 2V T is generated is shown in Figure 2. In this figure transistors ^-K^ are equivalent to those in Figure 1 and the output voltage is V 4 . The reference voltage ^-,-, = N^ is generated by resistor R and by transistors M Q1 , M Q2 , connected in series between voltage supply line DD and reference potential line. However, the reference voltage j ^p will not be exactly 2V T because of transistors M Q1 and M Q2 which are diode-coupled, across which the voltage will be: where I Q is the current through the transistors M Q1 and M 02 and K Q is their gain constant. However neither I Q nor K Q can be considered as having constant values since I Q depends on the supply voltage V DD and Q is a function of process parameters and temperature. In the circuit of Figure 1 and referring to equation (0) the current I 3 controlled by voltage V^ would be: This current will be fed to transistor M . To obtain a precise ratio of V 4 ,-y ecπi al to K- j Vrp the current I 3 must therefore be lowered by a value equal to: As shown in Figure 2, a current of this value can be subtracted from I 3 using additional transistors M 5 , M 6 and My. Transistors 5 and My are coupled in series between the ground reference potential line and the output of the current mirror composed of transistors M 2 and M 3 . The gate of transistor 5 is coupled the gate of transistor M-_ and the gate of transistor M 7 is coupled to the junction between transistors M Q1 and M Q2 . Transistor M g is coupled between the ground reference potential line and the input of the current mirror with its gate coupled to the gate of transistor My. Transistor M has a wide channel and acts as a voltage follower. Its output voltage V 5 is given by: The current I 5 through transistor M 5 operating in the triode region is: V 5- V 5 2 l which gives from equation (13 ) : I 5 = 5 ( V τ /ϊ^ + 3I o/Ko ) (14 ) By setting: K 5 =2x 1 gives : Now subtracting I 5 from I 3 gives : This is close to the required value of x - | _V T 2 but still requires the cancellation of the 2 I Q term in order to achieve very high precision for the ratio This can be achieved by adding to current 1^ a current I 6 flowing through transistor Mg. By setting K 6 = 2^ then: I 4 =x [I-L+Igj-I-xK-L (17) Current I 4 flowing through transistor M 4 now has the required value and generates a voltage: if its gate is connected to its drain or: if its gate is connected to the gate of the transistor M^ The above description refers to an embodiment of the circuit according to the invention in which voltages are generated whose value is proportional to the threshold voltage of the n-channel transistors. To generate voltages proportional to the threshold voltage of the p-channel transistors a circuit complementary to that described above may be used.

Claims: Claims 1. A voltage source circuit comprising: a current mirror having an input and an output and coupled to a first reference potential line; a reference current source coupled to the current mirror input; and a bias transistor having a first current electrode coupled to the current mirror output, a second current electrode coupled to a second reference potential line and a control electrode coupled so as to produce at its first current electrode a voltage dependent on the reference current, wherein said current mirror output forms an output of the voltage source circuit. 2. A voltage source circuit according to claim 1 wherein said reference current source.comprises a transistor having a first current electrode coupled to said current mirror input, a second current input coupled to said second reference potential line and a control electrode for receiving an input reference voltage. 3. A voltage source circuit according to claim 2 wherein said input reference voltage has a value of substantially twice the threshold voltage of the transistor forming the reference current source. 4. A voltage source circuit according to either claim 2 or claim 3 wherein the control electrode of said bias transistor is coupled to receive said input reference voltag . 5. A voltage source circuit according to either claim 2 or claim 3 wherein the control electrode of said bias transistor is coupled to said current mirror output. 6. A voltage source circuit according to claim 3 wherein said input reference voltage is produced at the gate electrode of a first diode-coupled transistor coupled via a second diode-coupled transistor to said first reference potential line. 7. A voltage source circuit according to claim 6 further comprising means for adjusting the currents at the input and output of the current mirror in order to correct the voltage at the output of the voltage source circuit. 8. A voltage source circuit according to claim 7 wherein the adjusting means comprises a first adjusting transistor coupled in series between said current mirror output and the first current electrode of a second adjusting transistor, the second adjusting transistor having a second current electrode coupled to said second reference potential line, and a gate electrode coupled to receive said input reference voltage and the gate electrode of the first adjusting transistor being coupled to the gate electrode of said second diode-coupled transistor, so as to subtract an adjusting current from the- current produced at the output of the current mirror. 9. A voltage source circuit according to claim 8 wherein the adjusting means further comprises a third adjusting transistor coupled between the current mirror input and the second reference potential line and whose gate electrode is coupled to the gate electrode of said second diode-coupled transistor so as to add an adjusting current to the current produced by the reference current source.

Inventors: RUSZNYAK ANDREAS, RUSZNYAK, ANDREAS
Applicants: MOTOROLA INC, MOTOROLA INC.
Application year: 1988
Main classification: G05F3
IPC codes: G05F3