Audio amplifier

An audio amplifier including a non-inverting amplifier stage that provides linear amplification without loop feedback for low distortion audio systems. The amplifier stage includes a triode vacuum tube or a triode connected pentode vacuum tube and a P channel field effect transistor (FET). The vacuum tube and the FET are connected in a series combination in which the source of the FET is connected to the cathode of the tube through a suitable resistance. The drain of the FET is connected to a negative voltage and both the gate of the FET and the grid of the tube are at DC ground potential. The input signal to be amplified is fed into the gate of the FET and the output is taken from the plate of the tube which is supplied with a positive voltage through a suitable load. In most applications the grid of the tube is connected to ground, however in some applications the grid may serve as an extra inverting input.

BACKGROUND OF THE INVENTION 
This invention relates to an audio amplifier and more particularly, to an 
audio amplifier that enjoys an improved performance when compared to 
conventional amplifiers, due to an improved linearity in various 
amplifying stages and a reduction in parisitic interactions. 
Some designers of current state-of-the-art audio amplifying equipment are 
becoming aware of a general lack of correlation between the subjective 
quality of the sonic performance of such equipment and the objective 
quality as measured by laboratory measuring instruments. For some years 
amplifying equipment for high fidelity applications has been designed 
around its laboratory performance based on a small number of classical 
steady state tests. One popular test which has gained wide acceptance 
because of the simplicity involved is the total harmonic distortion test. 
This test checks for modifications done to a single frequency in the 
equipment under test. An actual music signal is composed of a large number 
of frequencies however, and equipment that has been designed to pass a 
single frequency or a simple combination of frequencies under steady state 
conditions may not pass a complex, constantly changing wave form with good 
fidelity. 
The traditional method of designing audio equipment involves connecting a 
number of high gain amplifying stages together and applying a negative 
feedback loop around the entire number of stages to produce the desired 
amplifier gain and total harmonic distortion characteristics. Total 
harmonic distortion figures measured in the laboratory are generally 
lowest and thus best for those amplifier designs having the largest amount 
of feedback. The gain stages in most of these types of amplifiers are 
designed for maximum gain with little attention paid to the linearity of a 
single isolated stage. 
The problems associated with these types of amplifiers are numerous. For 
example, the subjective quality of performance is generally poor. Also, 
the phase shift within the amplifier reacts with feedback and causes 
oscillation. In addition, a form of time delay distortion is produced when 
a correction signal is fed back through the feedback loop and amplifier 
after the signal that was to be corrected had already passed out of the 
amplifier. Transient intermodulation distortion is produced when fast 
rising waveforms interact with phase compensating networks and high 
gain--high feedback situations to completely saturate the input stages of 
the amplifier. Distortion produced from amplifier clipping is much more 
apparent in a high feedback amplifier for two reasons. First, the 
electronic devices that make up the amplifier are driven into heavy 
saturation for which recovery takes a relatively long time. Secondly, the 
clipping is sharper and more higher order harmonics are generated 
resulting in a harsher and more obvious clipping sound than would 
otherwise be generated. 
Although prior designs exist which do not use a large negative feedback 
loop, they are not without problems. For example, the active devices that 
make up the amplifying stages are somewhat nonlinear in their transfer 
characteristics thus producing unacceptable levels of harmonic distortion. 
Also, there are parisitic capacitances between the input and output ports 
on these active devices which restrict the frequency response of the 
amplifier and cause higher loading of the source at high frequencies. In 
addition, the DC voltage levels can drift due to temperature changes and 
changes in component values thus restricting the useful dynamic range of 
the amplifier. 
SUMMARY OF THE INVENTION 
It is therefore, an object of the present invention to provide an amplifier 
which overcomes the above mentioned problems. 
It is also an object of the present invention to provide an amplifier stage 
which is inherently linear so that harmonic distortion is low. 
It is a further object of the present invention to provide an amplifier 
stage with well controlled gain which can be used as a basic building 
block for a complete amplifier without the need for feedback. 
It is a still further object of the present invention to provide an 
amplifier that has excellent subjective audio performance. 
It is a still further object of the present invention to provide an 
amplifier stage that has good high frequency performance with minimum 
phase shift so that the tendency to oscillate is reduced if feedback is 
applied around a number of such stages. 
It is a still further object of the present invention to provide an 
amplifier stage that is not dependent on feedback for maintenance of good 
frequency response. 
It is a still further object of the present invention to provide an 
amplifier which does not load the driving source substantially at high 
frequencies due to parasitic capacitances. 
It is a still further object of the present invention to provide an 
amplifier that does not have time delay distortion. 
It is a still further object of the present invention to provide an 
amplifier free from transient intermodulation distortion. 
It is a still further object of the present invention to provide an 
amplifier in which clipping is much less noticeable when the amplifier is 
overloaded. 
It is a still further object of the present invention to provide an 
amplifier in which the DC voltage levels are stable over wide temperature 
ranges and reasonable variations in circuit component values. 
Toward the fulfillment of these and other objects, the amplifier stage of 
the present invention comprises a P channel field effect transistor (FET) 
having a source electrode, at least one gate electrode, and a drain 
electrode; circuit means for connecting a gate electrode in series with a 
signal source; a triode or triode connected pentode vacuum tube having at 
least a cathode electrode, a grid electrode and a plate electrode; circuit 
means for connecting the source of the FET to the cathode of the tube 
through a suitable resistance; circuit means for connecting the grid of 
the tube to ground or to a source of signal modifications voltage such as 
feedback voltage as an inverting input; circuit means for extracting an 
output signal developed across a suitable plate load; and a circuit means 
to connect the drain of the FET to a negative voltage source.

DESCRIPTION OF THE PREFERRED EMBODIMENT 
As shown in FIG. 1, a P-channel field effect transistor (FET) 10 is coupled 
through a bias resistance 12 of appropriate value to a triode vacuum tube 
14 to form the basic amplifying element. The FET 10 has a gate electrode 
16, a drain electrode 18, and a source electrode 20. The tube 14 has a 
grid electrode 24, a cathode electrode 26, and a plate electrode 28. 
The gate electrode 16 of the FET 10 is connected to an input signal source 
30 which provides the signal that is to be amplified. The drain electrode 
18 of the FET 10 is connected to a source of negative voltage 32. The 
source electrode 20 of the FET 10 is coupled through the bias resistance 
12 to the cathode electrode 26 of the tube 14. The grid electrode 24 of 
the tube 14 is connected to ground 34 by an appropriate circuit means. An 
output 36 of the amplifier stage is developed on the plate electrode 28 of 
the tube 14 across a suitable load impedance 40 which is connected to a 
positive voltage source 42. The load 40 may take the form, for example, of 
a resistance across which the output voltage is developed, or a winding of 
a coupling transformer or the like. 
In some applications a signal may be fed into the grid electrode 24 of the 
tube 14 to modify the signal being amplified which is fed into the gate 
electrode 16 of the FET 10. This may be done by disconnecting the grid 
electrode 24 from ground 34 as shown in FIG. 2, reconnecting the grid 
electrode 24 to a terminal 44 which may then be connected to a source of 
signal modifying voltage. The grid electrode 24' input, which now becomes 
a secondary input, is an inverting input and may be used, for example, to 
provide for negative feedback. The primary signal input at the gate 
electrode 16 of the FET 10 is a non-inverting input; that is, the output 
signal that appears on the plate electrode 28' of the tube 14' is in phase 
with the input signal that is fed to the input of the amplifier. 
It is understood that the tube 14 is provided with a heater or filament and 
suitable voltage for heating the cathode 26 in a conventional manner. 
The FET 10 can be of the junction gate type or of the insulated gate type 
(MOSFET), the only requirement being that it is a P channel device. The 
FET 10 may also be of the type that has two or more gates which can be 
connected to extra voltage sources without departing from the scope of 
this invention. 
The vacuum tube 14 can be of the triode, pentode, or other multi-grid type, 
the only requirement being that it is connected as a triode by connecting 
extra grids through appropriate circuit means to either the plate 
electrode or the cathode electrode. 
It is further understood that this invention provides an amplifier stage 
which may be part of a multistage amplifier. The amplifier stage described 
herein may be used single ended, in push-pull pairs with a common load, 
cascaded in series, or used in conjunction with other types of amplifier 
stages to form a complete amplifier without departing from the basic 
invention. 
The advantages of the above circuit are numerous. The combination of the 
FET and the tube when connected together as an amplifying element is 
substantially more linear than either the FET or the tube if used by 
themselves. This provides a means by which audio amplifiers can be built 
without the need for feedback loops to control harmonic distortion. An 
amplifier designed with little or no feedback using this above described 
circuit avoids many other types of distortion such as time delay and 
transient intermodulation distortion. 
Another advantage of the present invention is the complete isolation of the 
input signal from the output signal. In either a tube or a transistor 
there is a capacitance between the input and the output such as between 
the grid and the plate of a tube. A portion of the output voltage is fed 
back to the input because of this capacitance, and since the input signal 
is out of phase with the output signal in most configurations, the high 
frequency performance of the circuit is poor. Furthermore, the signal 
source is loaded more at high frequencies. In the present design, the 
plate to grid capacitance does not feed any part of the output signal back 
to the input, but rather to ground provided the grid is grounded. There is 
no such feedback in the FET either because the drain is at signal ground, 
and the source voltage is in phase with the gate signal voltage and 
comparable in magnitude. 
A still further advantage of the amplifier stage of the present invention 
is that it is stable with respect to temperature variations and component 
part variations. It is a self-biasing circuit and the input can be biased 
to DC ground potential with a high value resistor connected from the gate 
of the FET to ground. The performance of the circuit is not strongly 
dependent on the drain voltage, the only requirement is that the FET 
remain in its pentode region of operation. If the grid of the tube is to 
be used as a second input for signal modification purposes, the same 
biasing technique can be used for the grid as was used for the gate of the 
FET, i.e., a high value resistor to ground, as both the grid of the tube 
and the gate of the FET are high impedance inputs at DC ground potential. 
A still further advantage of the amplifier stage of the present invention 
is that it gives excellent subjective performance when used in an audio 
amplifier for amplifying music in a high fidelity reproduction system. 
The circuit of the present invention is suitable in both small signal, low 
power circuits as well as for power applications as long as the FET and 
the tube are selected for the particular application. 
It is understood that although the above description relates to a single 
channel, or mono, system, it is equally applicable to multi-channel 
design, such as a stereophonic reproduction system. 
Of course, variations of the specific construction and arrangement of the 
amplifier disclosed above can be made by those skilled in the art without 
departing from the invention as defined in the appended claims.