Hello,
I think there are questions you need to answer for yourself: "Are you doing this for test purposes?" or "Am I trying to model something and give it as an input to my circuit, or am I modelling a circuit that is doing this behavior?"
If you are doing it for test purposes or if you're going to use this as a stimuli, no you shouldn't use transistors opamps or anything that has a physical correspondence. They would just slow your simulations down and they're unnecessary complications. Also this is what many people would understand when you say I want to replicate a signal in simulation. At least that's what I thought. In this case you can write a verilogA model or just put two ideal switches, one connected to 8 VDC and the other connected to a sine wave of your choice. And then you can switch between them using a square wave signal. It's the most basic thing I can think of.
The second thing you might have asked for is the real circuit that can do this. The implementation of this circuit completely depends on your specifications and I don't think you have enough data to come up with solid specifications. For example it's easy to do this with a Wien oscillator with an on/off switch, but the problems are: "What is this circuit going to drive?" "How much accuracy do I really need?" "Do I really need to turn this off at a very weird voltage like 8V?"
I'm assuming you want the first one. For the second one, I can't design a circuit for you, but I can help you with a few problems you might face in the process. Depends on what you're trying to do really.