{"id":848,"date":"2012-01-19T07:45:13","date_gmt":"2012-01-19T15:45:13","guid":{"rendered":"http:\/\/rustybolt.info\/wordpress\/?p=848"},"modified":"2012-03-14T18:54:47","modified_gmt":"2012-03-15T01:54:47","slug":"2012-01-19","status":"publish","type":"post","link":"https:\/\/rustybolt.info\/wordpress\/?p=848","title":{"rendered":"2012-01-19 AGC Amplifier, FET and HV Transistor"},"content":{"rendered":"

\"\"<\/a>Leonard Meeks posted a schematic of\u00a0 Mk.7 Mod 2 AGC amplifier<\/a> to the Regenrx Yahoo group.\u00a0 I’ll include it here.<\/p>\n

The amp uses a FET for the input stage, a 2N3904 for the second stage of amplification and two high voltage transistors for the third and fourth stage.<\/p>\n

For now I will talk about the 2N3904 stage, but what I say also applies to the other two transistor stages.<\/p>\n

The base forward bias for the 2N3904 is furnished by an 8.2 megohm resistor.\u00a0 Assuming that the voltage across this resistor is 11 volts, the current through the resistor is about 1.34 microamps.\u00a0 Then assuming the current gain of the transistor is about 200, the collector current would be about 0.268 milliamps.\u00a0 If we multiply that current by the 39,000 ohm resistor, we get 10.45 V.\u00a0 Thus, there is almost 11V drop across the resistor, and the voltage at the collector is near zero.\u00a0 The transistor is almost into saturation and can no longer amplify.\u00a0 If the transistor’s gain happens to be higher, then since there is only 12V supply, the transistor will try to let as much current through the collector until the voltage across the collector to emitter is almost zero.<\/p>\n

The beta or current gain of a transistor may be 200 at room temperature but when it is at the other extremes of temperature the gain varies widely.\u00a0 At cold temperatures the gain falls, and at higher temperatures the gain increases.\u00a0 In this case, the circuit would most likely be in a receiver that uses tubes and the temperature would most likely be higher, and the current gain would increase.\u00a0 Thus this circuit would greatly suffer as the temperature rises.<\/p>\n

The text books do not recommend using this type of bias because of the disadvantages I’ve discussed.\u00a0 The easy way to compensate for the gain variations is to remove the top lead of the bias resistor from the supply and connect it to the collector of the transistor.\u00a0 As the current gain of the transistor varies, the collector voltage changes and feeds back a negative voltage to compensate.\u00a0 Some gain is lost, but the circuit is much more stable and less temperature sensitive.\u00a0 In this case, there are four stages of gain so the loss in gain would not be a problem.<\/p>\n

I’m not sure why the author connected the second to last stage to the 100V line.\u00a0 The last stage should have a voltage gain of greater than 10, in which case several volts input to its base would give a full 100 volt peak to peak output.\u00a0 So the circuit should work just fine with the second to last stage connected to the 12V supply.<\/p>\n

Update Jan 25\u00a0 The author seems to have taken my advice and redrawn the schematic<\/a>.<\/p>\n

Update Jan 29\u00a0 The author has posted a new drawing<\/a>.\u00a0 The label is missing from the collector load resistor of the MPSA42.<\/p>\n

The emitter or source resistors are not bypassed.\u00a0 This means the 2N3904 has a voltage gain of only 3 (6.8k \/ 2.2k).\u00a0 If bypass caps\u00a0 were added, there may be enough voltage gain that only 2 stages may be needed.\u00a0 But the base bias resistors don\u00b4t require an emitter resistor, so they could be zero ohms.<\/p>\n

I had another idea.\u00a0 The supply voltage is very high, over 100VDC.\u00a0 Instead of putting\u00a0the stages one after the other consecutively, this high voltage allows us to use\u00a0 them in a totem pole arrangement.\u00a0 The FET stage could be placed where the 2.2k emitter resistor is for the 2N3904.\u00a0 The .01 coupling capacitor would be removed from the drain and connected to ground.\u00a0 The 2N3094 becomes a grounded base amp.\u00a0 FETs are notorious for having a very wide spread of Idss, so the value of the FET\u00b4s 100 ohm resistor will have to be selected for the right drain current.<\/p>\n

If this arrangement is used, there may be enough gain to allow two stages to do the job.\u00a0 In that case, the FET could replace the emitter resistor of the MPSA42.<\/p>\n

 <\/p>\n","protected":false},"excerpt":{"rendered":"

Leonard Meeks posted a schematic of\u00a0 Mk.7 Mod 2 AGC amplifier to the Regenrx Yahoo group.\u00a0 I’ll include it here. The amp uses a FET for the input stage, a 2N3904 for the second stage of amplification and two high voltage transistors for the third and fourth stage. For now I will talk about the <\/p>\n

(Read More…)<\/a><\/p>\n","protected":false},"author":2,"featured_media":0,"comment_status":"closed","ping_status":"open","sticky":false,"template":"","format":"standard","meta":{"footnotes":""},"categories":[6,7],"tags":[],"class_list":["post-848","post","type-post","status-publish","format-standard","hentry","category-electronics","category-radio-frequency"],"_links":{"self":[{"href":"https:\/\/rustybolt.info\/wordpress\/index.php?rest_route=\/wp\/v2\/posts\/848","targetHints":{"allow":["GET"]}}],"collection":[{"href":"https:\/\/rustybolt.info\/wordpress\/index.php?rest_route=\/wp\/v2\/posts"}],"about":[{"href":"https:\/\/rustybolt.info\/wordpress\/index.php?rest_route=\/wp\/v2\/types\/post"}],"author":[{"embeddable":true,"href":"https:\/\/rustybolt.info\/wordpress\/index.php?rest_route=\/wp\/v2\/users\/2"}],"replies":[{"embeddable":true,"href":"https:\/\/rustybolt.info\/wordpress\/index.php?rest_route=%2Fwp%2Fv2%2Fcomments&post=848"}],"version-history":[{"count":18,"href":"https:\/\/rustybolt.info\/wordpress\/index.php?rest_route=\/wp\/v2\/posts\/848\/revisions"}],"predecessor-version":[{"id":1541,"href":"https:\/\/rustybolt.info\/wordpress\/index.php?rest_route=\/wp\/v2\/posts\/848\/revisions\/1541"}],"wp:attachment":[{"href":"https:\/\/rustybolt.info\/wordpress\/index.php?rest_route=%2Fwp%2Fv2%2Fmedia&parent=848"}],"wp:term":[{"taxonomy":"category","embeddable":true,"href":"https:\/\/rustybolt.info\/wordpress\/index.php?rest_route=%2Fwp%2Fv2%2Fcategories&post=848"},{"taxonomy":"post_tag","embeddable":true,"href":"https:\/\/rustybolt.info\/wordpress\/index.php?rest_route=%2Fwp%2Fv2%2Ftags&post=848"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}