Pseudo resistance circuits can be used to detect low frequency signals in charge detection circuits. Charge detection circuits have applications ranging from biological sensors to electronic sensors.
A charge detection circuit can be realized through a capacitor and resistor in parallel between an output terminal and an input terminal of an operational amplifier. However, in order to detect a signal having a comparatively low frequency (for example, about several mHz to several Hz), such as a biological signal, it is necessary to lower the cutoff frequency of a high-pass filter by a capacitor and a resistor. That is, it is necessary to increase the resistance value of the resistor (for example, to about 10 TΩ). When increasing the resistance value of the resistor, the use of a typical resistive element is not practical in terms of circuit size. Accordingly, a weak inversion region of a metal-oxide-semiconductor field-effect transistor (MOSFET) can be used to form a resistor having a large enough resistance value and small enough circuit size.
This technology is a pseudo resistance circuit capable of suppressing fluctuation in resistance value with fluctuation in processor temperature and facilitating adjustment. The pseudo resistance circuit includes a first MOSFET, a second MOSFET, a current source, and a voltage source. The use of MOSFETs instead of a typical resistive element allows the entire circuit to be smaller. In this pseudo resistance circuit, it is possible to suppress fluctuation in resistance value with fluctuation in process by the current source, which generates a current substantially proportional to absolute temperature and the voltage source, which generates a voltage in a substantially linear function of absolute temperature. Suppressing fluctuation in resistance value increases the bandwidth of this charge detection circuit by reducing the noise in the signal.
Increases signal detection bandwidth
Maintains small circuit size