A Very Low Level DC Current Amplifier Using Photocoupler Negative Feedback Circuit

Abstract

In order to improve the response speed of a very low level dc current amplifier, we propose a novel method in which a photocoupler, instead of a high resistor (1010 Ω), is employed for the negative feedback circuit. In this paper, it is shown experimentally that with the proper use of positive feedback, the rise time t r of 10 µs is obtained for this circuit at the current sensitivity of 10 pA. In comparison, the t r with the use of the high resistor with the shielding plate is 120 µs. (Here, the current sensitivity is the minimum measurable input signal current that produces an output signal voltage of a very low level dc current amplifier equal to its output noise voltage, or that leads to a signal-to-noise ratio of unity.)

Noise analysis of a very low‐level DC current amplifier using photocoupler negative feedback circuit

“…C o is the output capacitance to the ground of a reactance attenuator. To obtain a square-wave input current having high output impedance, a triangularwave voltage generator and a series-connected reactance attenuator (20 fF) are utilized in experiments [4][5][6][7][8]13]. For the simulation circuit, they are equivalently replaced by a square-wave current source i s with C o .…”

“…In this method, a grounded conductive plate was used to terminate the electric flux lines between the terminals of a high-ohmage resistor in order to decrease the stray capacitances, but the amplifier using a high-ohmage resistor with shielding method is not appropriate for miniaturization. To decrease its effective input capacitance and to speed up its output response speed, a positive feedback circuit was previously employed [4]. In this case, the amplifier was unstable and began to oscillate [5].…”

“…To alleviate these problems, an amplifier using a photocoupler negative feedback circuit was proposed [6]. In this method, some outputs of the amplifier are transformed into light by the light-emitting element of the photocoupler, then the received light is transformed into current by its light-receiving element, and is fed back to the input of the amplifier.…”

“…However, the method using positive feedback had the problem of an increase in the output noise of the amplifier [13]. In order to clarify the mechanism of noise generation of the very low-level DC current amplifier using a photocoupler negative feedback circuit, a circuit analysis was performed, noise models in the form of the electronic circuit simulator PSpice were made, and the simulation results were compared with the experimental results.…”

A Very Low Level dc Current Amplifier Using SC Circuit: Effects of Parasitic Capacitances and Duty Ratio on Its Output

“…Neither are the amplifiers with shielding methods appropriate for miniaturization. A positive feedback circuit [6] had also been used as another approach to decreasing the stray capacitances. The amplifier with the positive feedback circuit however is unstable and begins to oscillate in this case.…”