Input Impedance of OPA653 and OPA659 query.

Mohit Hada

Other Parts Discussed in Thread: OPA659, OPA653

Hi Everyone,

I want to Amplify( non-inverting) / buffer high speed sinusoidal signal of frequency upto 100MHz and Amplitude = 4V i.e. 8Vp-p without loading the other circuit components in shunt.

For the above purpose we chose high input impedance components such as - OPA 653 (Amplification) and OPA659 (Unity gain buffer).

With PSPICE Simulation, I am getting desired results but still I have theoretical doubt regarding the input impedance which is:-

Even though OPA653 has i/p impedance of 10^12 ohm || 2.5pf, and OPA659 has the same of 10^12 ohm || 1pF at DC. Will such a high value

sustain at 100MHz or will it fall in the order of few kilo ohms???

Circuit Image is inserted. Please give your views and thoughts.

Very Thankful in Advance.

over 11 years ago

0 Mohit Hada over 11 years ago

Intellectual 745 points

circuit image....

0 Hooman Hashemi over 11 years ago in reply to Mohit Hada

TI__Mastermind 22825 points

Hi Mohit,

At 100MHz, the 2.5pF input capacitance will reduce your input impedance to around 640ohm. The variation of the input capacitance with frequency will probably not impact you greatly.

Regards,

Hooman

0 Mohit Hada over 11 years ago in reply to Hooman Hashemi

Intellectual 745 points

Hi Hooman,

Thanks for the reply. According to you, any other part in the circuit in parallel to the input of the OPAMP will be loaded by the input impedance severly.

But I have one more doubt, If I have 2Vp-p signal @ 100MHz at the non-inverting input of say OPA659, then what will be the input bias

current at + terminal if the i/p impedance is so low ~ 640 ohms. Will the bias current become of the order of mA?

I am more confused now.

0 Hooman Hashemi over 11 years ago in reply to Mohit Hada

TI__Mastermind 22825 points

Hi Mohit,

The 2.5pF input capacitance needs AC current from whatever is driving the amplifier at 100MHz, but the device input bias current (a name usually reserved for the DC or average input current) remains very low for the JFET device.

Hope my explanation is clear.

Regards,

Hooman

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Input Impedance of OPA653 and OPA659 query.