home *** CD-ROM | disk | FTP | other *** search
/ NetNews Usenet Archive 1992 #31 / NN_1992_31.iso / spool / sci / electron / 21753 < prev    next >
Encoding:
Text File  |  1992-12-30  |  3.9 KB  |  102 lines
  1. Newsgroups: sci.electronics
  2. Path: sparky!uunet!clsi!kevin
  3. From: kevin@clsi.COM (Kevin Cameron)
  4. Subject: Re: Grounded Emitter Amplifier Query
  5. Message-ID: <1992Dec30.201143.406@clsi.COM>
  6. Originator: kevin@dubhe
  7. Sender: usenet@clsi.COM
  8. Reply-To: kevin@clsi.com
  9. Organization: CAD Language Systems Inc.
  10. References:  <1992Dec29.214231.1135@inmet.camb.inmet.com>
  11. Date: Wed, 30 Dec 92 20:11:43 GMT
  12. Lines: 88
  13.  
  14.  
  15. In article <1992Dec29.214231.1135@inmet.camb.inmet.com>, bwhite@cobra.camb.inmet.com (Bill White) writes:
  16. > Hello.  I am reading Horowitz and Hill's book on Electronics, but 
  17. > I am having trouble understanding the grounded emitter circuit.  In
  18. > particular, I am having trouble understanding why the grounded emitter
  19. > is not thermally stable.  I believe it, I just don't understand why
  20. > it is true.  
  21.  
  22.     -----+-------------+---------------------- V_{CC}
  23.              |             |
  24.              |             |
  25.              \             \
  26.              /             / R_{C}
  27.              \ R_1         \
  28.              /             /
  29.              |             |
  30.              |             |
  31.              |             +--------------------- V_{OUT}
  32.              |             | (collector)
  33.              |        +---------+
  34.              |        |         |
  35.       V_{B}->+--------+   npn   | (I've never seen a good
  36.              | (base) |         |  ascii transistor.)
  37.              |        +---------+
  38.              |             | (emitter)
  39.          \             |
  40.              /             |
  41.          \ R_2         |
  42.              /             |
  43.              |             |
  44.              |             |
  45.              |             |
  46.            -----         -----
  47.         ---           ---
  48.              -             -
  49.  
  50. It is true because the current gain of the device is very high. V_C needs to be
  51. set at ~ V_CC/2, if the transistor gain is 200 V_CC = 10V and I_C = 5mA, R_C is
  52. 5kOhm, then R_1 (assume R_2 is infinite for now) is ~9.4V/(5mA/200) = 376kOhm to
  53. bias it. A 1% change in gain gives a 1% change in I_C giving a voltage change of
  54. 50mV, 10% gives 0.5V. Considering that transistors are not supplied with a 
  55. gain guaranteed any better than 50% and that it varies considerably with
  56. temperature you can see why this is not a practical circuit (you assumed hFE was
  57. constant). Adding R_2 makes things worse since it makes the bias current vary
  58. more as Vbe changes.
  59.  
  60. Grounded emitter circuits are used mostly for AC amplification or switching.
  61. The circuit below has DC feedback courtesy of R_E to stabilize the biasing.
  62. At frequencies above a level defined by the R_E/C_E time constant the emitter
  63. is effectively grounded and the device operates at maximum current gain.
  64.  
  65. > 
  66. >         -----+-------------+---------------------- V_{CC}
  67. >              |             |
  68. >              |             |
  69. >              \             \
  70. >              /             / R_{C}
  71. >              \ R_1         \
  72. >              /             /
  73. >              |             |
  74. >              |             |
  75. >              |             +--------------------- V_{OUT}
  76. >              |             | (collector)
  77. >              |        +---------+
  78. >              |        |         |
  79. >       V_{B}->+--------+   npn   |
  80. >              | (base) |         |
  81. >              |        +---------+
  82. >              |             | (emitter)
  83.               \             +----
  84.                /             |    |
  85.             \ R_2         \    |
  86.                /             /   ___
  87.                |       R_E   \   ___ C_E
  88.                |             /    |
  89.                |             |    |
  90. >            --+-------------+----+---
  91. > 
  92.  
  93. Kev.
  94.  
  95. PS. The new "H & H" has an extended section on switch-mode power supplies.
  96.  
  97. -- 
  98. Kevin Cameron          CLSI, Suite 100, 5457 Twin Knolls Rd., MD 21045, USA
  99. kevin@clsi.com         Tel: (USA) (410) 992 5700 (- 5709) x224
  100. uunet!clsi.com!kevin   Fax:       (410) 992 3536
  101. -----------------------------------------------------------------------------
  102.