Patent GB-766919-A (GB)
Title: Improvements in or relating to device for producing an electric output signal which is a logarithmic function of an input current
IPC: G06G7/24
Filed: 1954-09-01
Abstract: 766,919. Voltage measurement; electric analogue calculating apparatus. BECKMAN INSTRUMENTS, Inc. Sept. 1, 1954 [Sept. 29 1953], No. 25384/54. Class 37. [Also in Group XL (b)] A device for deriving and indicating an electric output signal, which is a logarithmic function of a small input current (which may be derived from an ionization chamber) to be measured, comprising a vacuum diode, whose heater is under-run, through which the input flows as anode current to develop a voltage which is a logarithmic function of the input for application to the input of a direct voltage amplifier from the output of which heavy negative feed-back is applied in series with the diode anode-cathode voltage so that the amplifier output current is a logarithmic function' of the small input current and is measured and/or recorded as an indication thereof. In Fig. 2 an ionization chamber whose outer cylindrical electrode 11 is energized from a stabilized H.T. source 12 develops through its inner electrode 13 a small uni-directional current connected over contact 43 of selector switch S<SP>1</SP> in its normal position, current limiting resistance R<SP>1</SP> and the anode cathode circuit of electrometer diode 16 to earth. Capacitance C<SP>1</SP> loads the input to reduce errors due to variations of input connection capacitance while capacitance C<SP>2</SP> across the diode limits any H.F. transients of the input signal. The diode heater is energized from a regulated supply to operate at a low temperature of e.g. 850‹ K or less at which it is shown that the anode-cathode voltage is a logarithmic function of the input current over a range of 10<SP>-7</SP> to 10<SP>-13</SP> amps. and the anode-cathode voltage increment per decade current increase is 0.15 volts approx. The positive anode voltage is applied to the control grid of a tetrode 30 whose screen grid is supplied from bias potentiometer R<SP>16</SP> and whose anode is shunted to earth by a stabilizing low-pass filter network 34 the multiple sections of which have progressively differing time constants from e.g. “ sec. to 10 microsec., and is directly connected to the input control grid of a double triode 35, the sections of which are directly coupled over a common cathode resistance; the output anode being directly coupled over a constant voltage drop discharge tube 39 to a cathode follower triode 37 whose cathode load circuit comprises meter 23 indicating logarithmically the value of the input current, in series with a calibrating variable resistance R<SP>6</SP> and fixed resistances R', R<SP>8</SP> across the latter of which a recorder 25 may be connected. Negative feed-back from the amplifier output is applied to the diode cathode over resistance R<SP>3</SP> superimposed on a variable positive bias from potentiometer R<SP>5</SP>. To remove blocking due to the arrival of negative going switching or other transients at the input, a normally closed switch S<SP>2</SP> may be momentarily opened and closed either manually or automatically to charge and then discharge the capacitance C<SP>3</SP> through discharge tube T<SP>1</SP> to apply a positive pulse to the diode anode. Reverse discharge applying a negative transient is prevented by earthing the junction of the tube T<SP>1</SP> and capacitance C<SP>3</SP> over voltage limiting resistance R<SP>11</SP>, and the tube cathode is illuminated by tube T<SP>2</SP> continually energized from the H.T. supply to ensure consistent discharge in response to operation of switch S<SP>2</SP>. For adjustment of calibration, switch S<SP>1</SP> is variable to connect earthed contact 46 to the diode input so that potentiometer R<SP>16</SP> is adjustable for zero output, and thereafter the switch is variable to connect standard test currents derived over contacts 44, 45 and series resistances R<SP>14</SP>, R<SP>15</SP> from preset potentiometers R<SP>9</SP>, R<SP>10</SP> energized from the stabilized H.T. source, so that potentiometer R<SP>5</SP> is adjustable for appropriate meter readings. The diode 16, the tetrode 30, resistances Rl, R<SP>14</SP>, R<SP>15</SP> and capacitances C<SP>1</SP> and C<SP>2</SP> are placed in a thermostatically controlled enclosure and the correct diode cathode temperature is obtained by regulating the heater voltage until the appropriate anode voltage increment per decade of input current variation is obtained. The amplifier negative feed-back is effective to reduce the effective input capacitance and anode to cathode leakage of the diode circuit.