Abstract:
A voltage reference circuit is disclosed having a common gate differential stage which utilizes the base-to-emitter voltage V BE , of a bipolar transistor to provide a reference current through a first resistor. Current mirror means are coupled to the differential stage to couple the reference current to second and third resistors which develop the output reference voltage. By ratioing the second and third resistors to the first resistor, a stable output reference voltage which is proportional to the V BE  of the bipolar transistor is provided.

Description:
CROSS REFERENCE TO RELATED APPLICATION 
     Related subject matter can be found in the following copending application which is assigned to the assignee hereof: 
     U.S. Pat. No. 4,450,367, entitled &#34;Delta V BE  BIAS CURRENT REFERENCE CIRCUIT&#34;, filed Dec. 11, 1981, by Roger A. Whatley. 
     TECHNICAL FIELD 
     This invention relates generally to reference circuits, and, more particularly, to a circuit which provides reference voltages proportional to a base-to-emitter voltage, V BE . 
     BACKGROUND ART 
     A common voltage reference standard is the V BE  of a transistor. Although known reliable V BE  references exist, such voltage references are commonly implemented with at least one regulating operational amplifier which may not be efficient for all size and power considerations. 
     BRIEF SUMMARY OF THE INVENTION 
     It is an object of the present invention to provide an improved voltage reference circuit. 
     Another object of the present invention is to provide an MOS voltage reference circuit which is smaller and simpler than related circuits in the prior art. 
     Yet another object of the present invention is to provide an improved voltage reference circuit which may be easily implemented in integrated circuit form using a minimum of circuit area. 
     In carrying out the above and other objects and advantages of the present invention, there is provided, in one form, a voltage reference circuit having a bipolar transistor. A differential stage is coupled to the bipolar transistor and a first resistor to reflect the V BE  voltage of the bipolar transistor across the first resistor. A current mirror is coupled to the differential stage to mirror the reference current to second and third resistors coupled to the current mirror. An output reference voltage which is proportional to the V BE  voltage is developed across the second and third resistors. Tne above and other objects, features and advantages of the present invention will be more clearly understood from the following detailed description taken in conjunction with the accompanying drawing. 
    
    
     BRIEF DESCRIPTION OF THE DRAWING 
     The single FIGURE illustrates in schematic form a voltage reference circuit constructed in accordance with a preferred embodiment of the present invention. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
     Shown in the single drawing is a voltage reference circuit 10 comprised generally of reference voltage portion 12, reference current portion 14, differential stage portion 16, current mirror portion 18, a start-up portion 20 and an output portion 22. While specific N-channel and P-channel MOS devices are shown, it should be clear that voltage reference 10 could be implemented by completely reversing the processing techniques (E.G. P-channel to N-channel or by using other types of transistors. 
     Referring to the FIGURE, reference voltage portion 12 comprises a bipolar transistor 24 having the base and collector electrodes thereof connected together and coupled to a power supply voltage V DD . Reference current portion 14 comprises a resistor 26 having a first terminal coupled to power supply voltage V DD  and a second terminal. Differential stage portion 16 comprises P-channel MOS transistors 28 and 30. MOS transistor 28 has a source electrode connected to an emitter electrode of bipolar transistor 24 at node 32. A gate electrode and a drain electrode of transistor 28 are connected together to a gate electrode of transistor 30. MOS transistor 30 has a source electrode connected to the second terminal of resistor 26 at node 34. Current mirror portion 18 comprises N-channel MOS transistors 36 and 38. N-channel transistor 36 has a drain electrode connected to the source and gate electrodes of transistor 28 and to the gate electrode of transistor 30. N-channel transistor 38 has a drain electrode connected to its gate electrode. The drain and gate electrodes of transistor 38 are both connected to the drain electrode of transistor 30 and to the gate electrode of transistor 36. Output portion 22 comprises resistors 40 and 42. Resistor 40 has a first terminal coupled to the source electrode of transistor 38 and a second terminal coupled to a voltage node such as ground. Resistor 42 has a first terminal coupled to the source electrode of transistor 36 and a second terminal coupled to the ground voltage node. Start-up portion 20 comprises an N-channel MOS transistor 44 having a drain electrode connected to the source and gate electrodes of transistor 28 and to the drain electrode of transistor 36. Transistor 44 has a gate electrode coupled to a start signal and a source electrode coupled to the ground voltage node. The start signal (not shown) may be a momentary positive voltage signal sufficient to make transistor 44 conduct thereby supplying a current path through transistors 28 and 44 to ground and inducing current flow through transistors 30 and 38. 
     In operation, transistors 28, 30, 36 and 38 function as a common gate differential amplifier with first and second inputs at nodes 32 and 34, respectively. Bipolar transistor 24 establishes a reference voltage at node 32 which is approximately V DD  -(V BE  of transistor 24). The differential stage portion 16 functions to maintain the same voltage at node 34 which exists at node 32. The voltage across resistor 26 is therefore V BE . Resistor 26 establishes a reference current I which flows through transistors 30 and 38 and resistor 40. Therefore, bipolar transistor 24 functions as a reference voltage means and resistor 26 functions as a reference current means. When the gate dimensions of transistors 28 and 36 are sized substantially the same as the gate dimensions of transistors 30 and 38, respectively, substantially the same current I flows through transistors 28 and 36 and resistor 42. In this configuration, transistors 28 and 36 function as a bias current means for providing a constant bias current for bipolar transistor 24. Transistors 30 and 38 function as a bias voltage means for providing a bias voltage to the bias current means. The output reference voltage V.sub. OUT exists at the first terminal of resistor 40 and is substantially (R 40  /R 26 )V BE  volts where R 40  is the value of resistor 40 in ohms and R 26  is the value of resistor 26 in ohms. The same reference voltage is provided at the first terminal of resistor 42 (not shown) which is substantially (R 42  /R 26 )V BE  volts where R 42  is the value of resistor 42 in ohms. The value of resistance of resistors 40 and 42 must be substantially the same because transistors 36 and 38 require the same gate-to-source voltage, V GS , in order to operate. However, any output reference voltage of any desired proportionality to V BE  may be provided. 
     The function of transistor 44 is to start voltage reference circuit 10 by pulling the gate electrodes of transistors 28 and 30 toward ground voltage potential. Reference circuit 10 has two stable states of operation. The first state of operation is when no current is flowing through P-channel transistors 28 and 30 and the output reference voltage is at ground voltage potential. In this state, the gate electrodes of transistors 28 and 30 are biased at a positive voltage potential sufficient to make transistors 28 and 30 nonconducting. Such a voltage potential would be greater than the arithmetic sum of V DD  and the threshold voltage of P-channel transistor 28. The first state of operation may occur when the start signal is at any sufficiently low voltage potential to make transistor 44 nonconducting. The second state of operation is when current begins to flow through transistors 28 and 30. Reference circuit 10 will provide a stable output reference voltage once transistor 44 is made to conduct thereby inducing current flow through diode-connected transistor 28. Current flow through transistor 28 induces current to flow through transistors 30 and 38. Once an output reference voltage exists, reference circuit 10 is self-regulating and the start signal should be removed from the gate of transistor 44. 
     Although the output reference voltage of reference circuit 10 will contain a temperature coefficient, the initial tolerance to processing variation is low. The nominal value of the output voltage at 25° C. over processing will not vary greatly. This is because in conventional MOS processes the forward bias voltage of the base-emitter junction of transistor 24 is practically immune to ambient variations and process changes. The only susceptibility to variation of the V BE  of transistor 24 is a change in current flowing through transistor 24. However, the current fluctuations through transistor 24 due to process and temperature variations will not be of sufficient magnitude to cause significant forward bias voltage variations across the base-emitter junction. 
     While the invention has been described in the context of a preferred embodiment, it will be apparent to those skilled in the art that the present invention may be modified in numerous ways and may assume many embodiments other than that specifically set out and described above. Accordingly, it is intended by the appended claims to cover all modifications of the invention which fall within the true spirit and scope of the invention.