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Thevenin Theorem seems to not work in this 1 bit RAM RC circuit

  • Engineering
  • Thread starter Martel
  • Start date
  • #1
3
1

Homework Statement:

Statement provided as an image

Homework Equations:

Equations provided at the end of the post (latex and SageMath/Python)
I have a problem which consist in 1 bit RAM made of 3 MOSFETs. One of the questions is to calculate the maximum voltage that the memory element can receive. I have obtained the result by inspection (it is 4 Volts) but I'm unable to reach the same by applying the Thevenin Theorem.

My understanding is: I have a circuit made of several resistors and one capacitor (which is the memory element). If the circuit is reduced to a $ (V_{TH}, R_{TH}) $ Thevening Equivalent, and given that the capacitor behaves like an open circuit for long periods of time (it's fully charged), I can assume that the maximum voltage the capacitor might have is $ V_{TH} $. The problem is that the $ V_{TH} $ I find has nothing to do with the expected result (4 Volts).

**I want to know what I'm doing wrong and how to solve this problem by applying the Thevenin theorem.**

The exercise, along with all my schemas and equations, are below:

Screenshot from 2020-01-14 12-49-16.png


In a previous question I have calculated the parasitic resistance and it is $ R_{PA} = 185.347405560882 $ TeraOhms.

The question I actually need to answer is:
Now, suppose the drain of Q1 is high, and the store line is held at the same voltage as the drain of Q1 . What is the maximum voltage, in Volts, that the gate of Q2 can be charged to? Note, this value must be larger than VOH = 3.5 Volts to satisfy the static discipline.
Now, my 'Tehevening Equivalent' attempt to find the voltage in Q2 is below:

2.jpg


1.jpg


$$
parallel(R_{1}, R_{2}) = \frac{1.00000000000000}{\frac{1.00000000000000}{R_{1}} + \frac{1.00000000000000}{R_{2}}}
\\
R_{\mathit{TH}} = {\rm parallel}\left(R_{\mathit{PA}}, R_{\mathit{ON}} + {\rm parallel}\left(R_{\mathit{OFF}}, R_{\mathit{PU}}\right)\right)
\\
I_{\mathit{TH}} = \frac{V_{S}}{R_{\mathit{PU}} + {\rm parallel}\left(R_{\mathit{OFF}}, R_{\mathit{ON}} + R_{\mathit{PA}}\right)}
\\
e = -I_{\mathit{TH}} R_{\mathit{PU}} + V_{S}
\\
\mathit{ITH}_{2} = \frac{e}{R_{\mathit{ON}} + R_{\mathit{PA}}}
\\
V_{\mathit{TH}} = -\mathit{ITH}_{2} R_{\mathit{ON}} + e
$$

Or in `SageMath`/`Python`:

Equations in SageMath/Python:
V_S = 5.
R_ON = 2100
R_OFF = 110e6
R_PU = 10e3
R_PA = 185.347405560882e12 # Parasitic resistance

parallel(R1, R2) = 1./(1./R1 + 1./R2)

R_TH = parallel(R_PA, R_ON + parallel(R_OFF, R_PU))

I_TH = V_S / (R_PU + parallel(R_OFF, R_ON + R_PA))
e = V_S - I_TH * R_PU
I_TH2 = e / (R_ON + R_PA)
V_TH = e - I_TH2*R_ON
The final result (`V_TH`) I get is `4.99954549553765` Volts, and it should be 4 Volts.
 

Answers and Replies

  • #2
400
159
Homework Statement:: Statement provided as an image
Homework Equations:: Equations provided at the end of the post (latex and SageMath/Python)

I have a problem which consist in 1 bit RAM made of 3 MOSFETs. One of the questions is to calculate the maximum voltage that the memory element can receive. I have obtained the result by inspection (it is 4 Volts) but I'm unable to reach the same by applying the Thevenin Theorem.

My understanding is: I have a circuit made of several resistors and one capacitor (which is the memory element). If the circuit is reduced to a $ (V_{TH}, R_{TH}) $ Thevening Equivalent, and given that the capacitor behaves like an open circuit for long periods of time (it's fully charged), I can assume that the maximum voltage the capacitor might have is $ V_{TH} $. The problem is that the $ V_{TH} $ I find has nothing to do with the expected result (4 Volts).

**I want to know what I'm doing wrong and how to solve this problem by applying the Thevenin theorem.**

The exercise, along with all my schemas and equations, are below:

View attachment 255608

In a previous question I have calculated the parasitic resistance and it is $ R_{PA} = 185.347405560882 $ TeraOhms.

The question I actually need to answer is:


Now, my 'Tehevening Equivalent' attempt to find the voltage in Q2 is below:

View attachment 255609

View attachment 255610

$$
parallel(R_{1}, R_{2}) = \frac{1.00000000000000}{\frac{1.00000000000000}{R_{1}} + \frac{1.00000000000000}{R_{2}}}
\\
R_{\mathit{TH}} = {\rm parallel}\left(R_{\mathit{PA}}, R_{\mathit{ON}} + {\rm parallel}\left(R_{\mathit{OFF}}, R_{\mathit{PU}}\right)\right)
\\
I_{\mathit{TH}} = \frac{V_{S}}{R_{\mathit{PU}} + {\rm parallel}\left(R_{\mathit{OFF}}, R_{\mathit{ON}} + R_{\mathit{PA}}\right)}
\\
e = -I_{\mathit{TH}} R_{\mathit{PU}} + V_{S}
\\
\mathit{ITH}_{2} = \frac{e}{R_{\mathit{ON}} + R_{\mathit{PA}}}
\\
V_{\mathit{TH}} = -\mathit{ITH}_{2} R_{\mathit{ON}} + e
$$

Or in `SageMath`/`Python`:

Equations in SageMath/Python:
V_S = 5.
R_ON = 2100
R_OFF = 110e6
R_PU = 10e3
R_PA = 185.347405560882e12 # Parasitic resistance

parallel(R1, R2) = 1./(1./R1 + 1./R2)

R_TH = parallel(R_PA, R_ON + parallel(R_OFF, R_PU))

I_TH = V_S / (R_PU + parallel(R_OFF, R_ON + R_PA))
e = V_S - I_TH * R_PU
I_TH2 = e / (R_ON + R_PA)
V_TH = e - I_TH2*R_ON
The final result (`V_TH`) I get is `4.99954549553765` Volts, and it should be 4 Volts.
Unnecessary complications. DC steady state solution is not applicable here because the circuit is semi-dynamic latch.
In time scale of interest, Vg(Q2)=Vs-Vt=5-1=4V. If for some reason the voltage Vg(Q2) will rise above 4V, the Q3 can no longer be opened by "Store" signal.
 
  • #3
3
1
DC steady state solution is not applicable here because the circuit is semi-dynamic latch.
When you say DC steady state solution you mean that assuming that the capacitor will reach a steady state after a 'long' time is not correct in this circuit?

When you say 'semi-dynamic latch', do you refer to Dynamic logic?


In time scale of interest, Vg(Q2)=Vs-Vt=5-1=4V. If for some reason the voltage Vg(Q2) will rise above 4V, the Q3 can no longer be opened by "Store" signal.
Please could you clarify this? This statement, along with the 'semi-dynamic latch' one, makes me reach the following conclusion:

Having Q1 opened and Q3 closed, Q2 starts to charge. Since the voltage between 'Store' and 'Q2' is greater than V_T (1 Volt), Q2 keeps charging. At some point, Q2 (due to Q1 being charging it) reaches a voltage so that the voltage between 'Store' and 'Q2' is less than 1 Volt, so Q3 opens. Q2 starts to discharge through the parasitic resistor. At some point, Q2 reaches a voltage so that the voltage between 'Store' and 'Q2' is >= 1 Volts again, so Q3 closes and Q1 charges Q2, and everything starts again.

Is the above correct?
 

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