On the Quality Factor of Transistors for Video Amplifiers
Abstract Generated abstract
The paper proposes a figure of merit for selecting transistors in video amplifier design, analogous to the gain bandwidth figure used for vacuum tubes. It derives an expression for the transistor figure of merit in terms of current gain, base resistance, collector capacitance, and a relative inertia coefficient that depends on the load resistance, noting that only a specified load permits an exact value. Using a simplified average case, the paper tabulates figures of merit for several domestic transistors and compares them with values for broadband electron tubes. The comparison suggests that some contemporary transistors approach tube performance, particularly when their maximum possible figure of merit is considered.
Full Text
On the Figure of Merit of Transistors for Video Amplifiers
I. N. Pustynskii
(Presented by the scientific seminar of the Radio Engineering Faculty)
In designing vacuum-tube video amplifiers [1, 2], tubes are selected on the basis of their figure of merit, by which is meant the quantity
\[ D_l = K_{\omega}\cdot f_{gr} = \frac{S}{2\pi \cdot C_l}. \tag{1} \]
By analogy, it is advisable to introduce the concept of the figure of merit (or gain area) of a transistor, which may be taken as equal to
\[ D_T = K_{\omega}\cdot f_{gr} = \frac{\alpha_0}{2\pi r_b C_k(1+q)}, \tag{2} \]
where
\[ q = \frac{\tau_e}{R_n C_k(1:\beta_0)} \]
is the coefficient of relative inertia of the transistor in an amplifier stage,
\[ \tau_e \]
is the time constant of the current transfer of the base.
In contrast to the figure of merit of tubes, the transistor figure of merit depends on the load value \(R_n\) through the coefficient \(q\) (see 2). For \(q \ll 1\), the maximum figure of merit is
\[ D_{T\max} = \frac{\alpha_0}{2\pi r_b C_k}. \]
Taking, for simplicity, \(q = 1\) (since usually \(q = 0 \div 2\)), we obtain the averaged figure of merit of the transistor
\[ D_{T\,av} = \frac{\alpha_0}{4\pi r_b C_k}. \tag{3} \]
The exact value of the figure of merit, however, can be obtained only for a definite value of \(R_n\).
Table 1 gives the figures of merit of domestic transistors according to formula 3), and, for comparison, the figures of merit of electronic tubes [3].
Table 1
| Transistor type | P401 | P407 | P404 | P402 | P403 | — | — |
|---|---|---|---|---|---|---|---|
| $D_{t_{\mathrm{cp}}}$, MHz | 21.4 | 36.0 | 43.5 | 75.0 | 150 | — | — |
| Tube type | 6P9 | 6Zh5P | 6Zh1P | 6Zh9P | 6E5P | 6S3P | 6S15P |
| $D_l$, MHz | 85 | 89 | 122 | 253 | 276 | 390 | 596 |
It is evident from the table that a number of transistors already almost do not lag behind tubes, especially if their maximum possible figure of merit is taken into account.
REFERENCES
- Bonch-Bruevich A. M., The Use of Electron Tubes in Experimental Physics. State Publishing House of Technical-Theoretical Literature, Moscow, 1954.
- Mamonkin I. G., Pulse Amplifiers. Gosenergoizdat, 1958.
- Cherepnin N. V., Electron Tubes for Broadband Amplifiers. Gosenergoizdat, 1958.