1. Field of the Invention
The present invention relates generally to reference circuits, and more specifically to a bandgap reference circuit.
2. Description of the Related Art
A bandgap reference circuit is used to generate a precise and a stable output voltage. The generated voltage is independent of process, voltage, and temperature. The bandgap reference circuit is widely used in various analog and digital circuits that require a precise voltage for operation.
Where VEB3 is the emitter-base voltage of the bipolar transistor Q3, VT is the thermal voltage at room temperature, and N is the ratio of the emitter areas of the bipolar transistor Q2 to the emitter areas of the bipolar transistor Q1.
As can be seen from the equation (1), by adjusting the resistance ratio of resistor R2 to R1, the conventional bandgap reference circuit 100 can provide a stable reference voltage VOUT having a zero temperature coefficient. The voltage level of the voltage VOUT is at around 1.25V, which is approximately equal to the silicon energy gap measured in electron volts, i.e., the silicon bandgap voltage.
However, in order to meet the application requirement of different integrated circuits, a reference voltage with a substantially zero temperature coefficient at the lower voltage level is needed.
An aspect of the present invention is to provide a bandgap reference circuit to provide a reference voltage having a substantially zero temperature coefficient.
According to one embodiment of the present invention, the bandgap reference circuit comprises first, second, and third current sources, first and second operational amplifiers, first and second bipolar transistors, a feedback device, a voltage divider, and a first resistor. The first amplifier has a first input, a second input and a first output. The second amplifier has a third input, a fourth input and a second output. The first current source is coupled between a power supply node and the inverting input of the first amplifier. The second current source is coupled between the power supply node and the non-inverting input of the first amplifier. The third current source is coupled between the power supply node and the third input of the second amplifier. The first bipolar transistor has a base, an emitter coupled to the first current source, and a collector coupled to the ground voltage. The second bipolar transistor has a base coupled to the base of the first bipolar transistor, an emitter, and a collector coupled to a ground voltage. The first resistor is coupled between the second current source and the emitter of the second bipolar transistor. The feedback device is coupled between the third current source and the base of the second bipolar transistor. The feedback device is controlled by the second output of the second amplifier. The voltage divider divides a voltage difference between the third current source and the base of the second bipolar transistor to provide a reference voltage. The fourth input of the second amplifier is couple to one of the first input of the first amplifier and the second input of the first amplifier.
The invention will be described according to the appended drawings in which:
The current source unit 22 provides a plurality of stable bias currents I1, I2, and I3. In this embodiment, the current source unit 22 is a current mirror formed by a plurality of PMOS transistors M1, M2, and M3. Referring to
The bipolar transistor Q1 has a base configured to receive a bias voltage VB, an emitter coupled to the inverting input of the operational amplifier OP1, and a collector coupled to a ground voltage. The bipolar transistor Q2 has a base configured to receive the bias voltage VB, an emitter, and a collector coupled to the ground voltage. The resistor R1 is coupled between the non-inverting input of the operational amplifier OP1 and the emitter of the bipolar transistor Q2.
Referring to
Referring to
VD1=VD2=VB+VEB1=VB+VEB2+I2×R1 (2)
VEB1 is the emitter-base voltage of the bipolar transistor Q1, and VEB2 is the emitter-base voltage of the bipolar transistor Q2.
Accordingly, equation (2) can rearranged into the following equation (3):
Referring to
In this embodiment, the W/L ratio of the PMOS transistors M1, M2, and M3 in the current source unit 22 is set to 1:1:m, wherein m is a positive integer. Therefore, the currents I1 and I2 are substantially the same and the current I3 has m times the magnitude of the current I2.
For the purpose of conciseness, the voltage divider 24 composed of two series-connected resistors R2 and R3 is exemplified. However, the present invention is not limited to such a configuration. In this embodiment, the voltage divider 24 divides the voltage difference between the voltage VD3 and the voltage VB to provide a reference voltage VREF at the cross point of the resistors R2 and R3. Therefore, equation (3) can be rearranged into the following equation (4):
Since the emitter-base voltage of the transistor Q1 has a negative temperature coefficient and the voltage difference ΔVBE has a positive temperature coefficient, the temperature coefficient of the voltage VREF can be adjusted to be positive, negative, or substantially zero. For example, the positive temperature coefficient of the voltage VREF is obtained by increasing the value of m or increasing the resistance ratio of the resistor R4 to R1. The negative temperature coefficient of the voltage VREF is obtained by increasing the resistance of the resistor R3 of the voltage divider 24.
Referring to
In addition, the prior art bandgap reference circuit provides a stable reference voltage VOUT having a substantially zero temperature coefficient at around 1.25V. However, the bandgap reference circuit 200 of
The bandgap reference circuit 200 of
The above-described embodiments of the present invention are intended to be illustrative only. Numerous alternative embodiments may be devised by those skilled in the art without departing from the spirit and scope of the invention as recited in the following claims.
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