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發表於 2010-8-27 21:49:53
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回復 2# 的帖子
感謝大大提供的資料,這份資料好深奧,讓我感到頭痛...(英文程度極差)
先key 出來翻譯在說...不過怎麼有好多單字遇到換行都會加上'-' 這符號呢?
慢慢消化.....
順便送上KEY 好的內容
6.1.1 Gain measurement Curve tracer of bipolar transistor sets the base current for a commone-mitter configuration. Beta is then found by observing the resulting collector current. This is a poor bias approach for beta testing, as the magnitudes of the collector bias and signal cur-rents then vary with beta. Instead, it is better to measure beta at a spe-cific collector bias current and signal current. Beta is then determined from the resulting base current magnitude. Fixed collector bias and sig-nal currents are established in the beta test circuit of Fig. 6.1 by the feed-back around A1. In establishing these currents, A1 drives the transistor to maintain near-zero input voltage and current at the amplifier inputs. For zero input voltage, the commmon ends of R1 and R2 are at ground level,
圖片1
Fig. 6.1 Static and dynamic current gains are measured with this cir-cuit at bias and signal levels set by independent control potentiometers.
and the entire input signals are impressed on these resistors. For zero amplifier input current, the currents from R1 and R2 all flow in the collec-tor. The dc and ac collector currents are then Ic = V/R1 and Ic = Ei/R2, respectively.
The base current resulting from the above collector currents is sup-plied to A2 and flows in the feedback resistor R5. From this current an output voltage is generated equal to
算式1
where hFE is the dc or static value for beta and hfe is the ac or dynamic value. Using the last result, the two current gains are expressed in terms of the dc and ac components of eo, Eo and Eo, by
算式2
Amplifier A2 also serves as a convenient means of setting the collector-base bias VCB. From R3 a voltage is set at one amplifier input and is, thereby, set at the test transistor base. This base voltage establishes VCB, since the collector is held at zero voltage by A1. Then, from Fig. 6.1, VCB=-V1.
Using this circuit, low-frequency measurements can be made within the accuracy permitted by the temperature coefficient of beta. This tem-perature coefficient is about 0.5% .c near room temperature, and nor-mal ambient variations limit beta test accuracy to around 1 percent. Testaccuracy is also directly related to the accuracies of voltages V and Ei and of resistors R1,R2 and R5. At low currents an additional error in the test for hFE is the input bias current of A2, but this may be avoided by using an FET input operational amplifier. Further limitations on the hfe test are the amplifier bandwidth and stray capacitance shunting the high resistance of R5. These latter factors limit hfe testing to 10kHz or lower. By using a low test frequency with low-level tests, the stray ca-pacitance effects which often limit curve tracer measurement by hystere-sis looping are avoided. |
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發表於 2010-8-27 17:24:22
n_046| ！以 hFE 為例，有個數位式三用錶、計算機就可克難的搞定，當然可將它再理想化，單純探討設計者的想法也是很有趣。