US 3090011 A
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May 14, 1963 J. A. HAMMER LOGARITHMIC AMPLIFIER USING COMMON CATHODE POSITIVE COMPENSATING FEEDBACK Filed June 23, 1959 PRIOR ART 3 V U OUTPUT I m? Ru U0 INPUT U gRx I b UK u I OUTPUT Ub Uo I My This invention relates to an amplifier, the output voltage of which is logarithmically related to the input voltage.
Such amplifiers are for instance useful in computers for transposing a linear voltage analogue of a computer quantity into a logarithmic analogue, whereby a conversion into decibels can be elfected.
It is known that with suitable electron tubes, operated at appropriate conditions the grid current depends logarithmically from the grid-cathode and anode-cathode voltages.
For this purpose the tube can be used in a circuit according to FIG. 1 of the drawings, in which the anode of a tube V is connected to the positive terminal of a supply source b with a voltage U through an anode resistor R,,, the negative terminal of said source being connected to the cathode of the valve V through a cathode resistor R The output U is derived from the terminals 3 and 4. The grid of the tube V is connected to the input terminal -1 via a grid series resistor R and the input terminal 2 is directly connected to the output terminal 4. The potential between the input terminals 1 and 2 is indicated by U In this circuit the following relations apply:
From this it appears that log (U, U,, U )=log R,
s, 1., +(a U,-R,-t,-Rko,+i,)
in which U is the plate to cathode voltage of the tube V, U is the grid to cathode voltage of the tube, L; is the grid current, I, is the anode current, a is a constant generally equal to the reciprocal of the amplification factor of the tube, S is a factor generally known as the mutual conductance of the tube, and S is a factor generally known as the plate conductance of the tube. The term log signifies the logarithm of the following quantity.
It the circuit is arranged so that a=constant 1 S =constant (2) S =constant (3 e e U U U1 the equation will obtain the following form:
log U =C +C .i,,
which is a condition of the desired type.
It is possible to satisfy the conditions (1-4) by correctly adjusting a suitable tube-type.
In case U, can vary over a large range, e.g. 60 db which often occurs in practice, the condition (5 cannot be satisfied. Since the sum of U -l- U for the desired logarithmic operation is in the order of 1 volt, the minimum value Patented May 14, 1963 of U should be 10 volts for obtaining an accuracy of only 10%, in which case the maximum value of said potential would be 10,000 volts, which is impracticable.
By means of the circuit as described it will therefore not be possible to obtain a reasonable accuracy over a large range.
An object of the invention is to provide a circuit in which over a large range (60 db) a good accuracy 4 db) can be obtained.
It is an object of the invention to provide a logarithmic amplifier circuit, the output voltage of which is an accurately logarithmic function of the input voltage throughout a large range of input voltages.
A feature of the invention is the provision of means, adding to the input voltage such a voltage, derived from the amplifier, that the voltage losses across all components of the input circuit with the exception of the series resistor are compensated.
A further feature is that said means consist of a second tube, the cathode of which is directly connected to the cathode of the first tube, the anode being directly connected to the supply source and the control grid of which being connected to the anode of the first tube through a potentiometer.
According to the invention the error in a thermionic grid controlled logarithmic amplifier resulting from the change in voltage of the grid of the tube with respect to ground with change in input current is corrected by applying a voltage to the cathode of the tube that is substantially equal and opposite to said grid voltage change. The compensating voltage is preferably obtained from an amplifier responsive to the anode voltage of the logarithmic amplifier.
The invention now will be described with reference to the drawings in which- FIG. 1 is a circuit of a known logarithmic amplifier and FIG. 2 is an amplifier circuit according to the invention.
In FIG. 2 a second tube V is cathode-coupled to the first tube V. The quantities referring to the valve V are indicated with the same reference symbols as the corresponding ones of the valve V, but provided with a prime.
The anode of V is directly connected to the supply source b and the control grid is connected with the sliding contact of a potentiometer P, connected between the anode of the valve V and the terminal 4.
As for the valve V: S U,, +S U =i,, the voltage generated across the common cathode resistor by the anode current is proportional to the grid voltage U and therefore also proportional to the voltage at the sliding contact of the potentiometer P with respect to the terminal 4. The voltage at said sliding contact is proportional to the voltage between the anode of V and the terminal 4, the relative magnitude being adjustable by means of the potentiometer P. The voltage U being proportional to and opposite (in sense) to the grid voltage U,; of the tube V it follows that the voltage, generated by the tube V across the cathode resistance will be proportional to and opposite (in sense) to the grid voltage U,, of the valve V. By means of a suitable adjustment of the sliding contact of the potentiometer P it can be obtained that the voltage generated by the tube V across R will be equal but opposite in sense to the voltage, generated by the valve V across R minus the grid voltage U of the valve V.
In this case U -l-U =0, so that the condition (5) is satisfied, regardless of the magnitude of the input voltage.
By means of the circuit it has been possible to satisfy the desired logarithmic relation within narrow boundaries (1% db) over a large voltage range (60 db).
1. A logarithmic amplifier comprising an amplifier tube having a grid-cathode circuit and an anode-cathode circuit, said grid-cathode circuit including a series resistor connected to a source of positive signal voltage such that grid current generally proportional to the signal voltage flows in the grid-cathode circuit, a cathode resistor common to said circuits, and means responsive to the current flow in the anode-cathode circuit arranged to maintain the potential of the grid substantially constant as the grid current varies with the signal voltage, whereby said grid current is proportional to said signal voltage.
2. In a logarithmic amplifier comprising an amplifier tube having a grid-cathode circuit and an anode-cathode circuit, a cathode resistor common to said circuits, a resistor connecting the tubegrid to a positive signal voltage source such that the grid current is generally proportional to the signal voltage applied to the circuit, and an amplifier that is connected to the anode-cathode circuit to be responsive to current flow in said circuit and that is connected to said cathode resistor to apply a voltage to the cathode of the amplifier tube tending to maintain the grid of such tube at a constant potential as the grid current varies with the voltage applied to the grid circuit, whereby the grid current is proportional to the signal voltage.
3. A logarithmic amplifier comprising an amplifier tube having an input circuit that includes a resistor connected between a signal source and a grid of the tube, an output circuit connected to an anode of the tube, and a cathode resistor, said tube having an anode current that is substantially a logarithmic function of the grid current, and means responsive to said output circuit adapted to apply 4 a voltage to the cathode of the tube in an amount to cancel changes in potential at the grid of the tube resulting from changes in grid current, whereby the grid current is determined solely by said series resistor and the voltage from said signal source.
4. A logarithmic amplifier comprising an amplifier tube having a grid, an anode, and a cathode, a resistor connecting the grid to a source of signal voltages that is polarized to cause grid current flow, an output circuit, a cathode resistor connecting the cathode to return sides of the signal source and the output circuit, said tube having an anode current that is a logarithmic function of the grid current, and a second amplifier tube that is responsive to said output circuit and that applies a signal to the first amplifier to cancel any variations in the sum of the gridcathode and cathode resistor voltages resulting from signal voltages in the grid circuit.
References Cited in the file of this patent UNITED STATES PATENTS 2,162,878 Brailsford June 20, 1939 2,600,423 Nolle June 17, 1952 2,662,213 Vanderlyn Dec. 8, 1953 2,685,000 Vance July 27, 1954 2,877,348 Wade Mar. 10, 1959 2,903,524 Howell Sept. 8, 1959 OTHER REFERENCES Vol. 21, MIT Radiation Laboratory Series (Electronic Instruments), McGraW-Hill, 1948, pages and 126.
Linear-to-Logarithrnic Voltage Converter," Electronics, July 1953, pages 156 and 157.