~ Triode Cascode Amplifier Calculator ~

 

This calculator can be used to calculate the gain of a triode cascode amplifier with different valves provided µ and r_a for both valves V₁ and V₂ are known at the required operating current ~ V₁ is a common cathode amplifier with the input impedance of a common grid stage V₂ as its anode load R_LV1

The calculation takes into account additional loads on V₂ due to the input impedance of following stages and their bias components which can be 'lumped' as R_G at the output ~ The default values are for two sections of an E88CC with Ia ≈ 1mA at V_a = 90V per valve ~ Click here for other calculators

Circuit Variables for —	V1	V2
Internal anode resistances — ra		kΩ
Amplification factors — µ
Unbypassed cathode resistor — Rk		kΩ
Anode load resistor — R1		kΩ
Additional anode load — RG		kΩ    Load of next stage
Calculated results for V1
Total anode load for V2 — RL		kΩ  =  R1||RG
Anode load on V1 due to V2 — RLV1		Ω  =  (ra2+ RL) / (µ2+1)
Voltage gain at anode — AV1		V/V  =  µ1 RLV1 / (RLV1+ ra1+ (µ1+1) Rk)
Voltage gain		dB  =  20log(AV1)
Calculated results for V2
Voltage gain — AV2		V/V  =  AV1 RL (µ2+1) / (ra2 + RL)
Voltage gain		dB  =  20log(AV2)
Modified anode resistance — ra2'		kΩ  =  ra2 + (µ2+1) ra1
Output resistance at anode — ROut		kΩ  =  ra2'||RL

 

In practice the valve used for a cascode is often a double triode and assuming the 2 sections are good and thus identical the cascode can be considered as a single valve with µ' = µ(µ+1) and r_a' = r_a(µ+2) ~ The new values of µ' and r_a' give a valve similar to a pentode which ~ providing there is sufficient HT voltage ~ can often replace pentodes with lower noise and possibly lower distortion ~ See Cascode a.f. amplifier

The common grid or grounded grid section of the cascode V₂ has a high non–inverting gain which can extend to UHF ~ Grounded grid triode amplifiers are used to amplify VHF and UHF signals because the screening action of the grid both prevents Miller capacitance and stops positive feedback from anode to cathode

In the 1950s commercial dual triodes specially made for cascode amplifiers like the ECC84 ECC85 and ECC88 were produced ~ These have a screen between the 2 anodes to prevent signal coupling between V₂ anode and cathode when V₁ anode is connected to V₂ cathode ~ Some like the ECC84 have a triode grid connected internally to the inter–anode shield

There is a book about 'valve amplifiers' which states the only general purpose valve that was designed to work well in a cascode is the ECC88 / 6DJ8 or E88CC /6922 . . . . Try other valves, by all means, but do not expect the performance to be as good maybe the 5th or 6th edition will now correct this along with many other misleading statements

Almost any dual or single triodes can be used to make an audio cascode amplifier if screening and/or grid stoppers etc. are used to prevent positive feedback at RF ~ Other statements relating to cascode heater/cathode voltage rating and heater induced hum in low level audio amplifiers are valid and should be considered

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