JPS6230968A - Current-voltage conversion circuit - Google Patents

Abstract

PURPOSE:To obtain a better conversion efficiency while achieving a higher-speed operation, by providing a base earthed transistor on the output side of a fine current source and a base earthed transistor to apply a bias current thereto to obtain the differential output between both the transistors. CONSTITUTION:The output of a fine current source 1 is connected to the emitter of a base-earthed transistor 7 which selected the operating point properly by a bias current and the collector output thereof is connected to an input terminal 4 of a differential amplifier 2a. Furthermore, to compensate for the offset in the output by a bias current of the transistor 7, the collector output of a base- earthed transistor 8 with the same bias current is connected to the other input terminal 5 of the amplifier 2a. This allows the transistors 7 and 8 to accomplish a buffer action to block a pole generation in cooperation with the feedback resistance of the amplifier and the current source capacity thereby enabling a high-speed current-voltage conversion with a better conversion efficiency.

Description

[Detailed Description of the Invention] [Field of Industrial Application] The present invention converts a weak output current from a pin bottom diode, a photomultiplier tube, etc. into a voltage with a stable high conversion efficiency up to high frequencies. This invention relates to a current-voltage conversion circuit that can convert current to voltage.

[Conventional technology]

FIG. 2 shows an example of a conventional current-voltage conversion circuit.
1 is a photodiode, 2 is an operational amplifier, and 3 is a feedback resistor R.
f, 4 is one input terminal of the operational amplifier 2, 5 is the other input terminal of the operational amplifier 2, and 6 is the output terminal of the converted voltage Vo. Here, the current-voltage conversion coefficient is Vo/1p=-R
It is given by f.

However, in an actual circuit, there is an input capacitance C8 consisting of the junction capacitance of the photodiode, the wiring capacitance from the photodiode to the operational amplifier, etc., and this input capacitance Cs and the feedback resistance Rf cause the frequency to become fp = 2 πRf - Cs. Make a pole. If this pole frequency is within a range lower than the bandwidth of the operational amplifier 2, the phase margin of this current-voltage conversion system decreases, causing ringing or oscillation.

In this way, when a pole is generated at a low frequency due to the input capacitor C3, a compensating capacitor Cf (=j) is added in parallel to Rf to stabilize the operation. However, if this method is used, the bandwidth of the current-voltage conversion system will be narrowed, and the operating speed will be reduced.

[Problem that the invention seeks to solve]

SUMMARY OF THE INVENTION An object of the present invention is to provide a current-voltage conversion circuit in which the input capacitance of a minute current source cooperates with the feedback resistance of an operational amplifier to prevent poles from being generated at relatively low frequencies.

[Means for solving problems]

In the case of the current-voltage conversion circuit originally targeted by this invention, the conversion coefficient must be sufficiently large, so it is impossible to reduce the feedback resistance of the operational amplifier. It is unavoidable that the phase margin decreases. For this reason, there is almost no room for modification on the amplifier side, and in order to solve the above problem, a means to prevent the input capacitance and feedback resistance from working together between the amplifier side and the minute current source side is needed. It was considered effective to insert .

As is well known, in a grounded transistor, the emitter side input impedance is extremely low and the collector side output impedance is extremely high, so we focused on the fact that it can be used as a buffer from the micro current source side to the amplifier side.
We decided to insert a common-base transistor whose operating point was selected appropriately using a bias current between the minute current source and the amplifier. However, this alone causes an offset in the output due to the bias current, so connect the collector output of the common-base transistor to which a similar bias current is applied to the other input terminal of the operational amplifier (which was conventionally grounded). It was decided to operate the operational amplifier as a differential amplifier to compensate for the output offset.

〔Example〕

FIG. 1 shows an embodiment of the present invention, in which 2a is a differential amplifier, 7
8 is a common base transistor for inputting a minute current; 8 is a common base transistor for compensating for the output offset due to the bias current of transistor 7; 9.1
0 is for 1-Langistav, 8 for emitter bias (several M
Ω) resistor, 11 is an offset compensation resistor, and other symbols are the same as in FIG.

As a result of creating a circuit like this, the influence of the input capacitance due to wiring etc. actually does not affect the operational amplifier, and no poles occur within the operational amplifier band: Due to the imaginary short circuit of the operational amplifier, the load impedance of the transistor becomes zero. The high-frequency cutoff frequency due to the collector junction is high: The input impedance of the common-base transistor is low, and the influence of input capacitance (high-frequency cutoff due to input capacitance, fluctuation in operating characteristics due to changes in input capacitance) is small: Therefore, wiring capacitance The following effects were observed: , the negative effects of wiring length are less likely to occur.

〔Effect of the invention〕

As described above, according to the present invention, it is possible to obtain a current-voltage conversion circuit that is extremely stable and operates at high speed with good conversion efficiency using a relatively simple circuit.

[Brief explanation of the drawing]

FIG. 1 is a circuit diagram of an embodiment of the present invention, and FIG. 2 is a side view of a conventional current-voltage conversion circuit for a minute current source. 1-photodiode, 2-operational amplifier, 2a-differential amplifier, 3-feedback resistor, 4.5-input terminal of operational amplifier, 6-output terminal of converted voltage, 7.8-base common transistor, 9. 10 - Resistor for emitter bias, 11 - Resistor for offset compensation, vo - Conversion output voltage, +vb - Voltage for photodiode reverse bias, +
Vcc - circuit power supply voltage.

Claims (1)

[Claims] The output of the minute current source is connected to the emitter of a common-base transistor whose operating point has been selected appropriately using a bias current, and its collector output is connected to one input terminal of a differential amplifier, and the bias current of the common-base transistor is In order to compensate for the output offset due to 1. A current-voltage conversion circuit characterized in that a buffering action is performed to prevent pole generation due to the cooperation of the current-voltage conversion circuit.