1. Field of Invention
The present invention relates to technology to design a voltage reference circuit. More particularly, the present invention relates to a circuit for producing a voltage reference that is insensitivity with temperature.
2. Description of Related Art
Voltage reference has been widely used in an analog integrated device. Since all the sub-circuits in the integrated device are influenced by the voltage reference, it is crucial to provide a temperature-insensitive voltage reference. The stable voltage reference is required in various device circuits. For example, a converter needs a stable voltage reference, so as to precisely produce the desired voltage. The output errors can at least be significantly reduced.
The circuit to produce a voltage reference has been introduced since the past many years. Basically, a fixed energy gap, such as VBE, for a bipolar transistor is often used to design the circuit to produce the voltage reference. One of the various conventional device with voltage reference is shown in FIG. 1. In FIG. 1, a bipolar transistor 90 is used. The emitter electrode 90e is connected to the ground voltage. The collector electrode 90c is receiving a current source 92, in which the other terminal of the current source 92 is connected to the voltage source Vcc. The collector electrode 90c and the base electrode 90b are coupled together and also connected to a sum unit 94. In addition, thermal voltage VT is generated by a VT generator 96. The output voltage VT is then multiplied by a multiplier 98 with a factor of M, so as to obtain a voltage level of MVT. The voltage level MVT is then also input to the sum unit 94, so that a voltage output Vout is obtained, in which Vout=VBE(ON)+MVT. Since the VBE(ON) has negative temperature coefficient while VT has positive coefficient, the desired voltage reference insensitive to the temperature can be obtained by choosing appropriate factor M.
For this conventional device with the voltage reference, the voltage difference VBE of the bipolar device is used as the voltage reference. Since the conventional IC fabrication is based on the CMOS process and it is hard to combine a bipolar device in the standard CMOS technology, it is not a good design to have the voltage reference by using a bipolar device. Also and, temperature coefficient of bipolar device is not exactly constant, M can be chosen to set temperature coefficient of output voltage to zero only at one temperature.
In order to produce the stable voltage reference, the conventional band-gap circuit is also proposed. However, the band-gap circuit usually also include two bipolar transistors with different area, so as to adjust the voltage difference. Again, the CMOS process is not convenient to include the bipolar device process. Therefore, how to design a voltage reference circuit without using bipolar device is desired.
The invention provides a circuit for producing a voltage reference, which can be fabricated under the CMOS process and is insensitive to the temperature, so that the circuit can be easily fabricated with a stable voltage level without being affected by the environmental temperature.
As embodied and broadly described herein, the invention provides a circuit for producing a voltage reference insensitive to temperature. The circuit includes a current source unit, a voltage-difference creating unit, and a resistance ratio unit. The current source unit receives an input current source and also receives a feedback signal from the resistance ratio unit. The current source unit thereby produces a first current source and a second current source which are equal in current. The voltage-difference creating unit includes a first MOS device and a second MOS device to respectively receive the first current source and the second source, wherein the first MOS device and a second MOS device has a threshold voltage difference. The resistance ratio unit includes a first resistor and a second resistor coupled in cascade, wherein the threshold voltage difference is applied to the first resistor. By adjusting a ratio of the first resistor to the second resistor, the resistance ratio unit produces a voltage reference, which is also the feedback signal to the current source unit.
It is to be understood that both the foregoing general description and the following detailed description are exemplary, and are intended to provide further explanation of the invention as claimed.