JPS61102877A - Pre-amplifier for image pick-up - Google Patents

Abstract

PURPOSE:To reduce the feedback resistor noise of a pre-amplifier without read errors of signal charge by changing the value of feedback resistor of the pre- amplifier when lowering the read rate in accordance with the rate. CONSTITUTION:The feedback resistor of a pre-amplifier is a convertible one as shown by broken lines 55. A changeover switch (SW) that has the same number of contacts as the number of read rates, and the same number of resistors are equipped. When the read rate is changed, a most suitable resistor is selected by the changeover switch (SW). Like conventional cases, the value of the feedback resistor that excludes errors is decided in accordance with vertical and horizontal signal conductor capacities, the open loop gain of pre- amplifier, and the frequency of horizontal signal reading. For purpose of extending the storage time or others, when lowering the scan speed, the feedback resistor is changed by the changeover switch S1-S5 in the inverse proportion. In this manner, the thermal noise that occurs in the feedback resistor is reduced drastically.

Description

DETAILED DESCRIPTION OF THE INVENTION [Field of Application of the Invention] The present invention relates to an imaging device that varies a signal readout period, and particularly to a preamplifier for amplifying a signal of an imaging device.

[Background of the invention]

In imaging devices, increasing the accumulation time increases the signal amount and improves the sensitivity. Therefore, by lowering the scanning speed (signal readout speed) and increasing the signal accumulation time in the photoelectric conversion body, the S/N can be improved. There is an imaging device designed to obtain images with a good ratio.

FIG. 1 shows an example of the configuration of a conventional solid-state imaging device, and its operation will be explained.

The pulses produced by the shift register 10 cause the photodiodes P,1-P,. . , 4°. The charge accumulated in
MoS transistors T1.1-T,. . , 4°.

The signals are sequentially read out from the upper row to the lower row to the vertical signal lines 1.1 to 1,400. And every other line, Q-Q
The transistors 4110 are sequentially turned on by the horizontal shift register 11, and charges are sent to the preamplifier pixel by pixel from the vertical signal line capacitor CV through the horizontal signal line. The charge is discharged by the preamplifier input resistor R, (see Fig. 2), and the discharge current is converted to a voltage and amplified by a negative feedback amplifier consisting of an amplifier 15 having an amplification degree G and a feedback resistor Rt16. The signal passes through a low-pass filter 19 whose band is controlled according to the speed and is output as a video output.

12 is a horizontal and vertical shift register drive circuit, and a control circuit 13 controls the transfer rate and the like.

This control circuit 13 controls the bands of the shift register drive circuit 12 and the low-pass filter 19 according to the amount of light, etc., and improves the sensitivity.

Next, the noise generated by the preamplifier section 20 will be described.

As is well known, the noise In generated in the preamplifier 20 is expressed by the following equation.

...(1) In equation (1), the first item, 4, +B/R, is the thermal noise of the feedback resistor R, and the second item, 4kT・1π''C,,''R,
, B3 are noises generated by the input capacitance C1 of the preamplifier.

If the read speed is slowed down and the band is limited accordingly, the bandwidth B in equation (1) will become smaller, and the preamplifier input capacitance noise in the second term will be large because it is proportional to the 3/2 power of B. A wide reduction can be expected.

However, the thermal noise of the first item, feedback resistor R8, is 1/2 of B.
Since it is only proportional to the power of the power, it is not possible to expect a large reduction like the preamplifier input capacitance noise.

That is, when the scanning speed is slowed down, the feedback resistance noise in the first item of equation (1) becomes dominant in the noise generated in the preamplifier, so this must be reduced.

[Purpose of the invention]

The present invention has the function of lowering the read speed.

This was done for the purpose of reducing preamplifier feedback resistance noise, which becomes dominant when the readout speed of an image sensor is lowered in an image pickup device.

[Summary of the invention]

From equation (1) above, in order to reduce the noise I, the feedback resistor R
It turns out that 2 should be made larger. However, simply increasing R7 unnecessarily causes a serious problem of unread signals. This will be discussed next.

Like the preamplifier 20 in Figure 1, the input resistance of a negative feedback amplifier has an open loop gain G and a feedback resistance R.
1, where G〉)1 is expressed.

Signal readout of the MO3 type solid-state image sensor can be expressed by an equivalent circuit as shown in FIG. 2, assuming that the vertical signal output line capacitance is C7. 30 is a vertical signal line capacitor C9 in which Q signal charges are accumulated. This charge is transferred to the MOS switch 31
is discharged through the preamplifier input resistor R, 32. Due to this discharge, the charge Q of the vertical signal line capacitance C7 becomes
In order to prevent unread data from occurring, it must become almost 0 within one read cycle, and according to experiments, C9 should be reduced to 3.
It has been found that if pF and C are 30 pF and the read cycle is 140jls, R+ must be 500Ω or less. Now, let's assume that the preamplifier's open loop gain is a practical 60dB.

From equation (2), it can be seen that the feedback resistance must be less than 500Ω.

However, if the read speed is lowered, the read cycle becomes longer, and even if the time constant of R,, Cv is increased, there will be no unread data, and R8, can be increased, that is, R can be increased. This will be explained specifically.

For example, if the read speed is reduced to 1/4, R1,
can be quadrupled. That is, R.

can also be multiplied by 41 times, and from equation (1) above, the feedback resistance thermal noise becomes 1/4, resulting in a significant S/N improvement.

This example shows the case where the scanning speed is increased by 174 times, but
Now, when the scanning speed is set to 1/N1, Rt t”M
If it is doubled, the feedback resistance thermal noise is 1/M, and an image with excellent S/N can be obtained with no unread data and extremely low noise.

Therefore, in the present invention, the value of the feedback resistor R is changed according to the reading speed, so that no signal charges are left unread, and the feedback resistor noise is reduced.

[Embodiments of the invention]

An embodiment of the present invention will be described below with reference to FIG. Third
The figure shows the preamplifier of the present invention with 500 pixels and 400 pixels horizontally.
An example is shown in which the present invention is applied to an MO3 type fixed imaging device, and the same parts as in the conventional example are given the same reference numerals.

The difference from the conventional example is that the feedback resistor of the preamplifier is of a switch type as shown in the dotted line frame 55.

The parts other than 55 within the dotted line frame are the same as the conventional example, and the photodiode P
The signal charges generated at 1,1 to P5oo, 40o are MO
Vertical signals sequentially from above through S transistors Tl, 1 to T500°400! 1,1 to 1° 400, horizontal MO8) - transistor Q, ~Q4°. The signals are sequentially read out to the preamplifier 15, where they are current-voltage converted and amplified, and outputted through the low-pass filter 19.

As shown in the dotted line frame 55 in Figure 3, there are selector switches SW for the number of readout speed variable stages (5 stages in this example) and resistors R1 to R5 for the number of variable stages, and when changing the readout speed, the optimum resistance is provided. Select with changeover switch SW. This changeover switch SW can be made mechanically, but it is also possible to use eleven-dimensional switching.
Next, we will explain one example using analog switches. 6 Switches SWI to SW5 in Figure 6 are electronic switches called analog switches, which become conductive when a voltage is applied to the control gate G, and become non-conductive when the voltage is removed. . Therefore, if a voltage is applied to the gate of one analog switch attached to the resistor to be selected in FIG. 4, electronic switching can be realized.

Returning to FIG. 3, this switching will be explained in more detail.

Now, as with the conventional example, the vertical signal line capacitance cv is 3pF.

The horizontal signal line capacitance CO is 309F, the preamplifier open loop gain is 60db, and the horizontal signal readout frequency is 7.
Assuming that the frequency is 2 MHz, the feedback resistance that does not cause unread data is <500 Ω as described above. Now, in order to lengthen the bodhisattva time, the scanning speed is set to 1/2.1/3.1.
/4.115, the feedback resistance is increased by 2, 3, 4, or 5 times, ie, IMΩ, by operating the switches 81 to s5 in inverse proportion to the scanning speed as described above.

Switch between 1.5 MΩ, 2 MΩ, and 2.5 MΩ.

(Even if this is done, no unread data will occur as described above.) By doing this, the thermal noise generated by the feedback resistor can be reduced to 1/2 according to equation (1). '1/3.1/4°115, the signal amount can be reduced by 2 times or 3 times.
Since it is multiplied by 4 times, 4 times, and 5 times, it is possible to realize a large improvement in S/N.

In the above embodiment, five types of resistors are used to switch the scanning speed in five stages, but the number of stages can be changed depending on the number of switching stages of the scanning speed. Furthermore, when it is necessary to switch a larger number of resistors, by selecting a combination of multiple resistors, it is possible to obtain many types of resistance values with a small number of resistors.

For example, by combining five resistors R1 to R5 as shown in FIG. 4 and making them conductive, 92 stages can be switched.

Although the above embodiments have been described using solid-state image sensors, it goes without saying that the present invention can also be applied to image pickup tubes.

Table 1 1: ON O: OFF //: Meaning of parallel [Effect of the invention] As explained above using the embodiment, low readout can be achieved by switching the return resistance of the preamplifier according to the readout speed. At high speeds, feedback resistance noise, which is dominant, can be significantly reduced without leaving anything behind, resulting in a significant improvement in sensitivity.

[Brief explanation of drawings]

FIG. 1 is a circuit diagram showing a configuration example of a conventional variable readout speed MO8 type solid-state imaging device, FIG. 2 is an equivalent circuit diagram showing a signal readout model, and FIG. 3 is an embodiment of the present invention. FIG. 4 is a circuit diagram of a preamplifier section according to an embodiment of the present invention. 1.1~1,400...Vertical signal line, Q~Q4°. ...Horizontal MOS switch, T-work 2, ~T,. . , 4°. ...Signal readout MOS switch, Pl, 1 to P,
. . , 4°. ...Photodiode, 10...Vertical shift register,
DESCRIPTION OF SYMBOLS 11... Horizontal shift register, 12... Shift register drive circuit, 13... Control part, 15... Amplifier,
16...Feedback resistance, 17...Preamplifier input capacitance,
18...Preamplifier input resistance, 19...Variable cutoff frequency depth 1 Diagram border 2 ■ Atsushi 3 Diagram

Claims (1)

[Claims] In an imaging device that can vary the scanning frequency of the imaging device and vary the accumulation time of the optical signal of the imaging device, the feedback resistor of the feedback preamplifier that amplifies the output signal of the imaging device is set at the scanning frequency (readout frequency). A preamplifier for an imaging device characterized by switching in response to a change in.