Common cathode-gate cascade amplifier

The conventional method to obtain low PIM distortion is to widen the open-loop frequency of the amplifier. When the circuit is required to have both a certain open-loop gain and sufficient open-loop bandwidth, the "common cathode-gate cascade amplifier circuit" composed of electron tubes (the transistor corresponds to the coscode circuit) is a good choice.

Figure 1 is a driving circuit designed by the author following the general architecture of "common cathode - gate-ground cascade circuit" (the other channel is the same, and the power supply can be found in the article "Direct-coupled OCL Electron Tube Headphone Amplifier" in "Electronics News" No. 29, 2007). V1 works in the common cathode state. In order to improve the frequency characteristics of the common cathode amplifier circuit, R2 introduces sufficient current negative feedback at this stage. V2 works in the common gate state with excellent frequency characteristics. V1 and V2 are connected in series. In order to obtain good impedance matching, a cathode follower composed of V3 is inserted after V2. C5, R8, and R9 introduce parallel voltage negative feedback to control the closed-loop gain of the circuit and reduce the distortion of the entire circuit. When the open-loop gain of V1 and V2 is large enough, the closed-loop voltage gain of the circuit can be calculated according to R8/R9 (ignoring the influence of the output impedance of the front stage) (When R8=200kΩ, the circuit can be used as a 20dB front stage: when R8=680kΩ, it is a 30dB voltage driver, which can directly drive the Class A field effect tube power stage with 0dB no voltage gain to form a gallstone amplifier. The specific circuit of the power stage can be referred to the audio article of "Electronics News", which is omitted here. When R8=680kΩ, the circuit is measured The amount of feedback (including the insertion loss of the cathode follower) is only -4dB, which is quite shallow (if you think the open-loop gain of this circuit is too low, you can replace the resistor with two series resistors while keeping the DC resistance of R2 at 1kΩ, and connect an AC bypass capacitor to one of the resistors to reduce the negative feedback of the common cathode circuit at this stage and increase the open-loop gain of the entire circuit). If the subsequent OdB Class A field effect tube power amplifier stage chooses a configuration without large-loop negative feedback, it will be more conducive to reducing the PIM, IIM and TIM distortion of the entire amplifier).

V1 and V2 use the medium-u high-frequency voltage amplifier tube 6N3, with a working current of 3mA, taking into account both the tone and input dynamics. V3 uses 6N1, with a current of 5mA and an output impedance of 220Ω (the working state of the circuit has been set by the design and no debugging is required). The gate landing capacitors of C1 and C2 will highlight the coloring effect of the capacitors, so their quality should be paid attention to. The values ​​of other components are shown in the circuit diagram.

The amplifier (tube amplifier) ​​driven by the "common cathode-grid ground cascade circuit" can not only reproduce the powerful and large dynamic range of music, but also has the delicate and crystal clear high notes, which are also full of the unique charm of vacuum tubes. It is worth a try for audio enthusiasts.