Beam current control circuit

A beam amount of a diode gun type pick-up tube is controlled in response to an incident light amount. Such beam control is carried out by making an excess beam amount constant. The calculation of the excess beam amount is done by subtracting a control electrode current and a signal current from a cathode current in case of the diode gun type pick-up tube.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
This invention relates to a beam control circuit to be applied to an image 
pick-up tube, and particularly to a beam current control circuit for a 
pick-up tube having a diode gun. 
2. Description of the Prior Art 
It is known that there are two types of image pick-up tubes, in which a 
triode gun type image pick-up tube operates with lower control electrode 
potential than cathode electrode potential, while a diode gun type image 
pick-up tube operates with higher control electrode potential than cathode 
electrode potential. 
Namely, when the image pick-up tube having a triode gun is in operation, a 
negative potential bias of +30V.about.-100V is supplied to a control 
electrode relative to a cathode potential bias of about zero volts. On the 
other hand, when the image pick-up tube having a diode gun is in 
operation, a positive potential bias of +10V.about.+20V is supplied to a 
control electrode relative to the cathode potential bias of about zero 
volts. 
Accordingly, in case of the image pick-up tube having the diode gun, there 
flows a control electrode current under operating condition, while there 
flows no control electrode current in case of the image pick-up tube 
having the triode gun. 
As shown in U.S. Pat. No. 3,975,657, a beam control circuit applied to the 
image pick-up tube having the triode electron gun by using an excess beam 
control technique is known. As described in the above-mentioned U.S. Pat. 
No. 3,975,657, a beam current of the amount of a pick-up tube has to be 
dynamically controlled in response to an incident light amount in order to 
maintain high resolution image. 
The idea proposed in the above-mentioned U.S. patent is to keep an excess 
beam current (which can also be referred to as a return beam current) 
constant in any event. But this patent only describes such control circuit 
for the image pick-up tube having a triode gun.

An electron beam Ib departed from a cathode electrode 2 lands on a photo 
sensitive layer 3, while the magnitude of the beam current Ib is 
controlled by a bias potential applied to a control electrode 4. 
An output signal current Is from the photo sensitive layer 3 is amplified 
at a buffer amplifier 5, and then converted into an output signal voltage 
Es by a resistor b. Of course, the output signal current Is is also 
derived through a signal output terminal 7 for a video signal processing. 
IN a current path for a control electrode current IG, there is proivded a 
control transistor 8, and a DC operating voltage +B is supplied to a 
collector electrode of this transistor 8 through a resistor 9. 
The control electrode current IG is converted into a potential form at the 
resistor 8, then supplied to a subtractor 10, wherein a potential EK 
corresponding to a cathode current IK and the converted control electrode 
current IG are compared. 
Namely, an output from the subtractor 10 is expressed as 
##EQU1## 
Further, thus obtained signal 
##EQU2## 
and previously explained signal Es are compared at a subtractor 11, and 
from which a signal ER, corresponding to a signal current IR is obtained, 
wherein the current IR (return beam current) is, expressed as 
##EQU3## 
Further, another comparator 12 is provided for comparing the signal ER with 
a reference voltage EO, and an output from the comparator 12 controls the 
control transistor 8. Such reference voltage EO is supplied from the 
reference voltage source 13. A reference numeral 14 designates a resistor 
for converting the cathode current IK into the signal voltage EK. 
Under these circumstances, following relations are established: 
EQU IK-IG=.alpha.Ib (1) 
EQU IR+Is=Ib (2) 
EQU Gm.times.EG=Ib (3) 
wherein 
IR: excess beam current (return beam current) 
GM: mutual conductance 
.alpha.: constant 
From the equations (1) and (2), the excess beam current IR is calculated as 
##EQU4## 
Namely, the excess beam current IR can be obtained by making linear 
relations among the cathode current IK, the signal current Is, and the 
control electrode current IG, and accordingly, even though the amount of 
the incident light causes the changes of the signal current IS, the amount 
of the excess beam current IR is controlled to be a constant value by 
controlling the control electrode current IG. 
In the circuit block diagram as shown in FIG. 1, the output signal ER from 
the subtractor 11 corresponds to the excess beam current IR. 
If an amplification factor of the comparator 12, and a resistance of the 
resistor 9 are expressed as A and R, respectively, then following equation 
is established. 
##EQU5## 
Therefore, the excess beam current is also expressed as 
##EQU6## 
As the value of AxRxGm shows a certain finite value, the excess beam 
current IR is always smaller than the reference current IO. 
It will be apparent to those skilled in the art that many modifications and 
variations may be effected without departing from the spirit and scope of 
the novel concepts of the present invention.