Photodiode preamplifier circuit for a card reader system

Claim 2

An amplifier circuit including, in combination: an output transistor connected between a voltage supply terminal and a point of reference potential, said output transistor having an output terminal thereof which exists at one or the other of two binary levels to indicate the presence or absence of a hole or mark on a sense card, a control transistor connected between said output transistor and an input terminal, said control transistor responsive to a change in input voltage at said input terminal to change the conductive state of said output transistor, and reference level control means connected between one electrode of said control transistor and said input terminal and responsive to a change in voltage at said one electrode of said control transistor for producing an offsetting change in voltage at said control transistor; said offsetting change in voltage maintaining a substantially constant current through said control transistor and thereby maintains a constant DC voltage level at said input terminal with respect to a threshold voltage level of said output transistor.

Claim 3

The amplifier defined in claim 1 which further includes feedback means interconnecting the output of said output transistor to the input of said control transistor and providing a regenerative feedback signal to said control transistor for rapidly changing the conductive state of said control and output transistors.

Claim 4

The amplifier defined in claim 1 wherein said reference level control means includes: a first transistor interconnected between a reference voltage and said control transistor, the conductive level of said first traNsistor being controlled by the conductivity of said control transistor, and a second transistor connected between said first transistor and said input terminal and responsive to changes in the conductivity of said first transistor for stabilizing the voltage level at said input terminal.

Claim 5

The amplifier defined in claim 3 wherein said reference level control means further includes a capacitor connected to said second transistor for providing a current to a photodiode connected to said input terminal when the impedance of said photodiode is increased as a result of said photodiode sensing a hole or a mark on a sense card, said current from said capacitor producing a desired voltage swing at said control transistor to turn off said control and output transistors and produce a high binary voltage level at said output terminal.

Claim 6

The amplifier defined in claim 4 which further includes a resistor connected in parallel with said capacitor between two electrodes of said second transistor for providing a leakage path for said capacitor.

Claim 7

The amplifier circuit defined in claim 5 which further includes: a first resistor connected between said voltage supply terminal and the output electrode of said output transistor for producing voltage swings at said output terminal in accordance with the current into said output transistor, regenerative feedback means including a second, feedback resistor connected between said output electrode of said output transistor and the control electrode of said control transistor for providing a regenerative feedback signal to said control electrode of said control transistor, a third resistor connected between the output electrode of said control transistor and a point of reference potential and developing thereacross a voltage level which controls conductivity of said output transistor, a fourth resistor connected between the input electrode of said control transistor and said voltage supply terminal, the current through said fourth resistor controlling the conductivity of said first transistor in said reference level control means, and a fifth resistor connected between the control electrode of said control transistor at a point of reference potential, said fifth resistor providing a desired level of input impedance for said amplifier circuitry.

Claim 8

An amplifier defined in claim 6 wherein: said first transistor in said reference level control means has the control electrode connected to a variable reference potential for controlling the conductivity of said reference level control means and for controlling the sensitivity of said amplifier, the input and output electrode of said first transistor connected in series between the input electrode of said control transistor and the control electrode of said second transistor, and said second transistor having the input and output electrodes thereof connected in series between said voltage supply terminal and said input terminal and conducting current to said input terminal and through a photodiode which is connected to said input terminal.