Vacuum tube and MOSFET transimpedance amplifier

An improved transimpedance amplifier includes a common cathode-connected vacuum tube first stage, coupled to a common-drain-connected MOSFET second stage, and an optional common drain-connected MOSFET third stage. Preferably, capacitive feedback is coupled from the output of the second stage to the input of the vacuum tube stage, to control the transimpedance of the amplifier. The circuit takes advantages of the low stray inter-electrode capacitances, low delay and transit times of the vacuum tube and the high transconductance of the MOSFET second and optional third stages, in a way complementary to one another to form a high transimpedance amplifier whose characteristics are controlled almost exclusively by the feedback capacitance or other feedback impedance, rather than by gain and stray capacitance terms of individual devices. The resulting transimpedance amplifier is useful in a number of applications, for example, in audio preamplifiers and audio power amplifiers.

FIELD OF THE INVENTION 
This invention pertains generally to the field of electronic amplifiers, 
and specifically to amplifier designs having advantageous forward 
transimpedance characteristics. 
BACKGROUND OF THE PRIOR ART 
Electronic amplifiers having favorable forward transimpedance 
characteristics are very useful in numerous fields including audio, video, 
communications, computers and control systems. An ideal transimpedance 
amplifier would transform an input current to an output voltage by a given 
ratio. An ideal transimpedance amplifier would have infinite frequency 
response, slew rate, voltage gain, current gain, and forward 
transconductance; zero input impedance, output impedance, input bias 
current, and distortion; unlimited available output current; and 
transimpedance characteristics which are determined only by a feedback 
impedance. Unfortunately, all real amplifier designs fall short of ideal 
characteristics to one degree or another, and such shortcomings affect the 
suitability of particular amplifier designs for particular applications. 
Typical prior art transimpedance amplifiers use various combinations of 
bipolar transistors, connected in common emitter or emitter follower 
configurations, connected as a Darlington pair, or as a differential input 
amplifier stage. Depending upon the requirements for a particular 
application, such prior art circuits may suffice, or in some cases it may 
be possible to further improve them by adding additional stages and 
components, but at the disadvantage of added cost, complexity and in some 
cases distortion. In my prior U.S. Pat. application Ser. No. 905,841, now 
U.S. Pat. No. 4,801,893, I disclosed a series of improved transimpedance 
amplifiers using FET first stages followed by one or more bipolar stages. 
These amplifiers overcome most of the shortcomings of the above-noted 
prior art designs. 
The present invention provides a different approach to transimpedance 
amplifier design, one that provides even further performance gains as 
compared to the above-noted designs, while maintaining the advantages of 
simplicity, ruggedness and low cost. The present invention makes use of a 
combination of a vacuum tube and one or more MOSFET devices, in a unique 
combination which takes advantage of important characteristics of both 
devices. Various combinations of vacuum tubes and semiconductor devices 
have been used before, but not in the configuration used in this 
invention. Those configurations are not appropriate for an improved 
transimpedance amplifier. See, for example, U.S. Pat. No. 4,163,198, which 
is a form of a series connection of a p-channel FET and a triode. This 
circuit differs from the present invention in that it does not invert 
phase, and consequently it cannot be used as a transimpedance amplifier. 
SUMMARY OF THE INVENTION 
By providing a unique combination of a vacuum tube first stage and a MOSFET 
second stage, with an optional MOSFET third stage, the present invention 
provides for an improved transimpedance amplifier having wider bandwidth 
and wider power bandwidth than could be provided using either all vacuum 
tubes or all MOSFETs. The invention takes advantages of unique properties 
of small signal vacuum tubes, i.e., low inter-electrode capacitances and 
low transit and switching times, in combination with certain advantages 
provided by a MOSFET device, i.e., relatively high transconductance, to 
provide a high performance transimpedance amplifier. 
Various combinations of vacuum tubes and MOSFETs have been used before, but 
not in the configuration used in this invention. Those configurations are 
not appropriate for an improved transimpedance amplifier.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
The transimpedance amplifier of FIG. 1 includes a vacuum tube triode, 
generally indicated by reference number 10, and a MOSFET device, generally 
designated by reference number 30. A signal input terminal 11 connects to 
the grid of vacuum tube 10, and through a grid resistance 12 to signal 
ground 13. The plate of vacuum tube 10 connects through load resistance 14 
to a positive voltage supply +V.sub.cc. The cathode of vacuum tube 10 
connects through a cathode resistance 15 to signal ground. This cathode 
resistance is bypassed by capacitance 16. 
The plate of vacuum tube 10 is also connected via lead 17 to the gate 
electrode of MOSFET 30. The drain electrode of MOSFET 30 connects to 
signal ground 13, and the source electrode is connected to a lead 31. One 
branch of this lead connects to the output terminal 32. Another branch of 
lead 31 connects to feedback capacitance 20, the other side of which 
connects to the input terminal 11 and the grid of vacuum tube 10. A source 
resistor 33 is connected from lead 31 to a source of positive voltage 
+V.sub.cc. Resistors 12 and 15 provide for grid and cathode biasing, 
respectively. Resistor 14 provides for the plate output loading. Resistor 
33 provides for source output loading. 
Vacuum tube 10 is connected in common cathode configuration, and as such 
provides voltage gain, with wide frequency response and wide power 
bandwidth. This advantageous result is obtained because the vacuum tube 
has low stray inter-electrode capacitances, and low delay and transit 
times. However, the vacuum tube stage by itself has relatively low 
transconductance. MOSFET 30 is connected in common drain configuration, 
and at this stage complements the first stage by providing high 
transconductance. The resulting combination circuit of FIG. 1 has high 
transimpedance, high transconductance, high voltage gain, and high current 
gain. Because of these factors, the transimpedance characteristics are 
determined almost exclusively by the feedback impedance capacitor 20, 
instead of being determined by the gain and stray capacitances of 
individual devices, as is often the case with prior art transimpedance 
amplifiers. 
It should be noted that in general, under some circumstances it might be 
desirable to operate the circuit without any feedback impedance, i.e., by 
deleting capacitance 20. In other cases it may be desirable to have some 
generalized feedback impedance connected from the source of MOSFET 30 to 
the grid of vacuum tube 10, other than a simple capacitance. In the 
preferred embodiment shown, capacitance 20 provides for high-frequency 
lead phase compensation, which stabilizes the circuit containing the 
present invention against high frequency oscillations. 
FIG. 2 shows a variation of the circuit of FIG. 1, including a third stage 
comprising MOSFET 40. The first two stages of the amplifier of FIG. 2, 
including vacuum tube 10 and MOSFET 30 and associated components, are 
identical to the circuit of FIG. 1, and individual components are given 
the same reference numbers. The additional, third stage consists of MOSFET 
40 connected in common drain configuration, with the drain electrode 
connected to signal ground 13. The source electrode connects through 
source output load 43 to +V.sub.cc. The output terminal 42 connects from 
the source electrode of MOSFET 40. 
As in the case of the embodiment of FIG. 1, the feedback impedance, which 
in the preferred embodiment is a capacitor 20, connects from the source of 
second stage MOSFET 30 to the grid of first stage vacuum tube 10. Other 
types of impedances could be used, depending upon the particular 
application. The third stage provides additional current gain so that the 
transimpedance characteristics of the overall amplifier are determined by 
feedback impedance, capacitor 20, and are relatively independent of any 
loading connected to the output of the circuit. 
The circuits of FIGS. 3 and 4 provide audio phonograph preamplifiers with 
RIAA equalization. They are suitable for receiving a signal from a 
phonograph cartridge, providing amplification and equalization, and 
providing an output signal for driving an audio power amplifier or other 
audio component. The circuits of FIGS. 3 and 4 are similar, but the 
circuit of FIG. 3 uses the two-stage transimpedance amplifier of FIG. 1, 
while the circuit of FIG. 4 uses the three-stage transimpedance amplifier 
of FIG. 2. Those skilled in the art will recognize that by using different 
feedback networks, different types of preamplifiers can be constructed. 
In FIG. 3, the transimpedance amplifier comprises vacuum tube triode 110, 
MOSFET 130, and associated components. The input of this transimpedance 
stage is at lead 111, which connects to the control grid of vacuum tube 
110. A grid resistor 112 connects to signal ground, 113. The cathode of 
vacuum triode 110 connects through cathode resistor 115, which is bypassed 
by cathode capacitance 116. The plate of triode 110 is connected to lead 
117. It also connects through load resistor 114 to a source of operating 
voltage. Lead 117 couples signals from the plate of vacuum triode 110 to 
the gate electrode of MOSFET 130. The drain electrode of this MOSFET 
connects to signal ground, lead 113. Source resistor 133 connects from the 
source electrode to a source of positive operating potential, described 
below. Lead 131 is connected to the source electrode of MOSFET 130, which 
provides the signal output of the transimpedance amplifier, and of the 
entire phono preamplifier in this case. Feedback capacitance 120 for the 
transimpedance amplifier connects between lead 131 and the input lead 111 
to the grid of vacuum triode 110. An input amplification stage is provided 
in front of the transimpedance amplifier, in the form of another vacuum 
triode 150. In practice, vacuum triodes 150 and 110 can be separate 
triodes in the same dual triode vacuum tube, type 12AX7. The input to the 
overall phono preamplifier of FIG. 3 can be provided between terminals 151 
and signal ground 113. A load resistor 152 connects across this input, and 
a coupling capacitor 153 connects from input terminal 151 to the grid of 
triode 150. The cathode of triode 150 connects through a cathode resistor 
154 to a node 155. A further resistor 156 connects from node 155 to signal 
ground. The plate of triode 150 connects through load resistor 157 to a 
source of positive operating voltage, described below. The plate also 
connects via lead 158 and series capacitor 159 to the input lead 111 of 
the transimpedance amplifier. 
As previously mentioned, the output lead 131 for the transimpedance 
amplifier is also the output of the overall phono preamplifier and is 
connected to output terminal 132. A portion of the equalization network is 
connected in the form of feedback from this output to the phono 
preamplifier input stage 150. Specifically, a network consisting of 
resistors 161, 162 and capacitors 163, 164 is connected between lead 131 
and the feedback lead 160. Feedback lead 160 connects to the previously 
mentioned node 155, and from there through resistor 154 to the cathode 
electrode of vacuum tube triode 150. 
A source of suitable operating potential may be applied to bias terminal 
170. A network including resistors 171, 172 and capacitors 173, 174 
connect from terminal 170 and supply filtered operating voltage to load 
resistor 157 of input triode 150, load resistor 114 of transimpedance 
triode 110, and the source resistor 133 of the transimpedance MOSFET 130. 
The phono preamplifier of FIG. 3 offers several advantages as compared with 
other types of amplifiers. These include lower distortion, better 
frequency response, and higher slew rates. This preamplifier also offers a 
reduction of output hum and better rejection of 60 Hz hum and other power 
supply noise and transients. These improvements are due to the ability of 
the transimpedance amplifier circuit to behave in a manner closer to the 
ideal than transimpedance amplifiers of the prior art, particularly in 
that its transimpedance characteristics are more nearly determined by the 
feedback impedance exclusive of other influences. In addition, this 
amplifier offers lower output impedance and higher current drive, allowing 
it to directly drive loads such as an equalization network without the 
need for a separate buffer amplifier. 
The overall configuration of FIG. 4 is in many respects the same as FIG. 3, 
and the same reference numbers are used for the corresponding components 
in FIGS. 3 and 4. Such components in FIG. 4 perform essentially the same 
function as previously described with respect to their counterparts in 
FIG. 3, and therefore such description will not be repeated. Individual 
circuit component values may vary, however, for bias and equalization 
purposes as will be apparent to those skilled in the art. The main 
difference in FIG. 4 over the circuit of FIG. 3 is the inclusion of a 
third stage in the transimpedance section. Specifically, this is in the 
form of MOSFET 140 and associated connections. MOSFET 140 is connected in 
common drain configuration with its drain electrode connection connected 
to signal ground 113. Lead 131, from the source and signal output of 
MOSFET 130, connects via lead 141 to the gate of MOSFET 140. The source of 
MOSFET 140 connects to a lead 142. Source resistance 143 connects from 
this lead to the source of operating potential. The output of the 
transimpedance amplifier is thus at lead 142, and this connects through 
the equalizing network (resistors 161, 162 and capacitors 163, 164) to the 
feedback lead 160, as previously described. Lead 142 connects through 
series resistor 166 and capacitor 167 to output terminal 169 for the 
overall phono preamplifier. An output load resistor 168 connects from 
terminal 169 to signal ground. 
The phono preamplifier of FIG. 4 offers all the advantages of the FIG. 3 
amplifier as compared with other types of amplifiers. In addition, it 
provides further improvements in frequency response, slew rates, reduced 
sensitivity to device characteristics, as well as even lower output 
impedance and higher current drive. 
Another use of the transimpedance amplifier of the present invention is as 
an interstage driver in an audio power amplifier. In FIG. 5, the 
transimpedance amplifier consists of vacuum tube triode 210, MOSFET 230, 
and associated components. Specifically, the input to the transimpedance 
amplifier is at lead 211, which connects to the grid of triode 210. As 
before, triode 210 can be one-half of a dual triode vacuum tube such as a 
12AX7. The output of the transimpedance amplifier stage is at lead 231, 
which is the source of MOSFET 230. Other components of the transimpedance 
amplifier stage include grid resistor 212, cathode resistor 215, bypass 
capacitor 216, interconnection lead 217, feedback capacitance 220, and 
plate load resistor 214. The source load resistor for MOSFET 230 includes 
a pair of resistors which also help set the bias conditions for the output 
transistors. Variable resistor 275 connects from lead 231 to a lead 276, 
and a resistor 280 connects from there to a source of positive potential. 
Together these two resistors form the source load for MOSFET 230. 
An input stage is provided for the power amplifier of FIG. 5, including 
triode 250 which, for convenience, can be the other half of a dual triode 
vacuum tube that also includes triode 210. The input terminal 251 for the 
power amplifier couples through series capacitor 253 to the grid of triode 
250. Input resistor 252 connects from the input to signal ground 213. The 
cathode of triode 250 connects through cathode resistor 254 to a node 255. 
Resistor 256 connects from this node to signal ground. The plate of triode 
250 connects through load resistor 257 to the source of operating 
potential, and also through a lead 258 and series coupling capacitor 259 
to the input 211 of the transimpedance amplifier. 
As previously mentioned, the output of the transimpedance amplifier stage 
is at lead 231. This is coupled through gate resistor 293 to MOSFET power 
output device 291. The signal is also coupled through variable resistor 
275 and bypass capacitor 281 to gate resistor 292 of power output MOSFET 
device 290. The drain of device 290 is connected to a source of positive 
operating potential 294, and the source of device 290 is connected to a 
lead 295. The source of device 291 is also connected to lead 295, and the 
drain of device 291 connects to signal ground. 
Lead 295 connects through series capacitor 296 and parallel-connected 
resistor 297 and inductance 298 to the power amplifier output terminal 
299. A feedback path is coupled from lead 295 through resistor 261, lead 
260, node 255, and resistor 254 to the cathode of input triode 250. 
Resistor 266 and capacitor 268, and resistor 267 and capacitor 269 provide 
two paths from lead 295 to signal ground, so as to stabilize power output 
MOSFET devices 290 and 291 against high frequency oscillations. Resistor 
282 connects from lead 283 to signal ground. 
The power supply for the input amplifier 250 and the transimpedance 
amplifier is applied to a power terminal 270 and distributed by the 
filtering network which includes resistors 271, 272 and capacitors 273, 
274. 
In operation, signals applied to the input terminal 251 of the power 
amplifier are initially amplified by the stage including triode 250, then 
are further amplified by the transimpedance amplifier, which then drives 
the output power devices 290, 291. 
The power amplifier of FIG. 5 offers several advantages as compared with 
other types of amplifiers. These include lower distortion, better 
frequency response, and higher slew rates. This power amplifier also 
offers a reduction of output hum and better rejection of 60 Hz hum and 
other power supply noise and transients. These improvements are due to the 
ability of the transimpedance amplifier circuit to behave in a manner 
closer to the ideal than transimpedance amplifiers of the prior art, 
particularly in that its transimpedance characteristics are more nearly 
determined by the feedback impedance exclusive of other influences. In 
addition, because the transimpedance amplifier offers lower output 
impedance and higher current drive, it can directly drive the output 
circuitry without the need for a separate buffer amplifier. 
The power amplifier of FIG. 6 is the same as the power amplifier of FIG. 5, 
with the addition of a third stage to the transimpedance amplifier. 
Otherwise, all components are the same in the amplifiers of FIGS. 5 and 6, 
and are given the same reference numbers in both Figures. 
The added third stage includes MOSFET 240, which is connected between 
MOSFET 230 and the output circuitry of the power amplifier. Specifically, 
the drain of MOSFET 240 is connected to signal ground 213, and the gate 
electrode is connected via lead 241 from the source electrode of MOSFET 
230. Resistor 233 is connected as the source load resistor for MOSFET 230, 
connecting to the source of operating potential. Variable resistor 275 and 
resistor 280 are connected as the source load resistance for MOSFET 240. 
As before, these resistors also set the offset bias for output devices 
290, 291. 
The power amplifier of FIG. 6 operates in the same manner as previously 
described for FIG. 5, except that the addition of the third stage, i.e., 
MOSFET 240, in the transimpedance amplifier provides additional isolation 
and current drive for the output stages. 
The power amplifier of FIG. 6 offers all the advantages of the FIG. 5 
amplifier as compared with other types of amplifiers. In addition, it 
provides further improvements in frequency response, slew rates, reduced 
sensitivity to device characteristics, as well as even lower output 
impedance and higher current drive. 
While the above examples have used vacuum triodes, it will be appreciated 
that other types of vacuum tubes could also be used. 
Thus it will be seen that the present invention has provided an improved 
transimpedance amplifier not heretofore recognized in the art, and which 
provides improved performance compared to prior art designs. The invention 
accomplishes this by taking advantages of unique properties of vacuum 
triodes and MOSFETs, and combining them in a unique way so that these 
characteristics complement one another to achieve the improved 
performance.