Talk:Common collector

message: The previous circuit and the corresponding expressions were too complicated to be useful (IMHO). They're much simpler now without all the biasing details. I do think its important to show a practical circuit though, so I'd like to add another section maybe showing the previous circuit and explaining the biasing (the simple formulas will still be valid once you account for loading). Roger 03:55, 30 May 2007 (UTC)
thread_title:  Simpler circuit and expressions 

message: I think the small-signal characteristics should be grouped together in a table. I don't have any experience with wiki-tables, but I just came up with this:
Current gain      	
{A_\mathrm{i}} = {i_\mathrm{out} \over i_\mathrm{in}} = \beta_0 + 1 \approx \beta_0 \quad (\beta_0 \gg 1)
Voltage gain      	
{A_\mathrm{v}} = {v_\mathrm{out} \over v_\mathrm{in}} = {g_m R_\mathrm{E} \over g_m R_\mathrm{E} + 1} \approx 1 \quad (g_m R_\mathrm{E} \gg 1)
Input resistance: 	
r_\mathrm{in} = r_\pi + (\beta_0 + 1) R_\mathrm{E}\ \approx \beta_0 R_\mathrm{E} \quad (g_m R_\mathrm{E} \gg 1 \wedge \beta_0 \gg 1)
Output resistance:	
r_\mathrm{out} = R_\mathrm{E} \| {r_\pi + R_\mathrm{source} \over \beta_0 + 1} \approx {1 \over g_m} + {R_\mathrm{source} \over \beta_0} \quad (\beta_0 \gg 1 \wedge r_\mathrm{in} \gg R_\mathrm{source})
How does it look (I know its a bit jumbled)? Can someone help me improve it? Roger 03:44, 30 May 2007 (UTC)
replies: text: here is my suggestion (Alessio Damato 08:40, 15 June 2007 (UTC)
replies: text: That looks better, thanks. If no one else has any ideas I'll start incorporating that one. Roger 16:41, 18 June 2007 (UTC)

text:  Very good concept. — Omegatron 20:11, 5 August 2007 (UTC)

text:  I think we should include the simpler approximations in the text itself, though, and segregate the more specific models to their own section, as I've tried to do to this one.  (Or maybe give each transistor model its own article?) — Omegatron 22:35, 5 August 2007 (UTC)

text:  I think using RE in the gain formula is misleading. RE is the emitter resistor, while in the formula we need RE parallel with Rload.  Also, the approximation for Av=1 is true for the criteria mentioned in the table. However, the whole idea of the CC circuit is to drive very low impedance loads, because it has a low output impedance, allowing for high output power. Now if you try this circuit with matched load (Rload=rout) then you'll notice the Av becomes 0.5 (ideally, in reality it will be somewhat different, but still much less than one). This is not mentioned in many pages and books discussing this circuit, I don't know why... (of course in reality the push-pull variation is used for power amps, but has the same concept) Hoemaco (talk) 12:17, 24 April 2020 (UTC)
thread_title:  How does this table look? 

message: How about this one?
Current gain      	
{A_\mathrm{i}} = {i_\mathrm{out} \over i_\mathrm{in}} = \beta_0 + 1	
\beta_0 \quad (\beta_0 \gg 1)
Voltage gain      	
{A_\mathrm{v}} = {v_\mathrm{out} \over v_\mathrm{in}} = {g_m R_\mathrm{E} \over g_m R_\mathrm{E} + 1}	
1 \quad (g_m R_\mathrm{E} \gg 1)
Input resistance: 	
r_\mathrm{in} = r_\pi + (\beta_0 + 1) R_\mathrm{E}\	\beta_0 R_\mathrm{E} \quad (g_m R_\mathrm{E} \gg 1 \wedge \beta_0 \gg 1)
Output resistance:	
r_\mathrm{out} = R_\mathrm{E} \| {r_\pi + R_\mathrm{source} \over \beta_0 + 1}	{1 \over g_m} + {R_\mathrm{source} \over \beta_0} \quad (\beta_0 \gg 1 \wedge r_\mathrm{in} \gg R_\mathrm{source})
I just put the approximate expression in a different column. I could also put the definition of each function in a separate column (e.g. a column with only Av = Vo/Vi next to "Voltage gain"). Comments? Roger 03:54, 30 May 2007 (UTC)
replies: text: here is my suggestion (Alessio Damato 08:40, 15 June 2007 (UTC)
replies: text: That looks better, thanks. If no one else has any ideas I'll start incorporating that one. Roger 16:41, 18 June 2007 (UTC)

text:  Very good concept. — Omegatron 20:11, 5 August 2007 (UTC)

text:  I think we should include the simpler approximations in the text itself, though, and segregate the more specific models to their own section, as I've tried to do to this one.  (Or maybe give each transistor model its own article?) — Omegatron 22:35, 5 August 2007 (UTC)

text:  I think using RE in the gain formula is misleading. RE is the emitter resistor, while in the formula we need RE parallel with Rload.  Also, the approximation for Av=1 is true for the criteria mentioned in the table. However, the whole idea of the CC circuit is to drive very low impedance loads, because it has a low output impedance, allowing for high output power. Now if you try this circuit with matched load (Rload=rout) then you'll notice the Av becomes 0.5 (ideally, in reality it will be somewhat different, but still much less than one). This is not mentioned in many pages and books discussing this circuit, I don't know why... (of course in reality the push-pull variation is used for power amps, but has the same concept) Hoemaco (talk) 12:17, 24 April 2020 (UTC)
thread_title: Alternative

message: Rout is deficient by the absence of re (the internal resistance of the emitter). This is equal to 25/Ie ohms where Ie is in mA. This should be added to the expression for Rout for correctness.  —Preceding unsigned comment added by 88.109.121.209 (talk) 00:27, 7 October 2007 (UTC)
replies: text: "re" is equal to 1/gm (where gm is IC/VT), so the current expression is indeed correct (look in the "	Approximate expression" column). -Roger 06:35, 7 October 2007 (UTC)
thread_title:  Error in expression for Rout 

message: Does anyone else think the derivation section might be a bit too detailed? I'm thinking about pruning it down. -Roger (talk) 20:49, 13 May 2010 (UTC)
replies: text: Some people don't want to see any math; I'm the opposite.  If there are any duplications or errors they should be fixed or deleted; but that doesn't seem to be the case.  IMHO the derivation section is a good example of what most electronics article should include.  No need to prune from something that is well done and correct.  Zen-in (talk) 00:12, 14 May 2010 (UTC)
replies: text: I get what you're saying, but at the same time the derivations are fairly straight forward and easy to find in a textbook (which Wikipedia is not). Perhaps we could include them in a collapsible side panel (I remember seeing something like that in some other article) instead? -Roger (talk) 01:02, 14 May 2010 (UTC)
thread_title:  Derivation section 

message: The article explains that "In this circuit the base terminal of the transistor serves as the input, the emitter is the output, and the collector is common to both (for example, it may be tied to ground reference or a power supply rail), hence its name".
This doesn't make sense to me. Can somebody explain what is meant by common?
ICE77 (talk) 01:21, 6 August 2015 (UTC)
thread_title:  Emitter follower naming reason 

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